diff --git a/INSTALL.md b/INSTALL.md
index d97665e..13926e4 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -1,97 +1,113 @@
-# Installation
+Installation
+============
 
-### Prerequisites
+Prerequisites
+-------------
 
-LinApple uses **CMake** as its build system.
+LinApple uses __CMake__ as its build system.
 
-#### Debian / Ubuntu / RetroPie
+    #   Debian / Ubuntu / RetroPie
+    sudo apt update
+    sudo apt install git g++ cmake \
+        libzip-dev libcurl4-openssl-dev zlib1g-dev imagemagick
+    sudo apt install libsdl3-dev libsdl3-image-dev      # Debian 13+
 
-```bash
-sudo apt-get update
-sudo apt-get install git g++ cmake libzip-dev libsdl3-dev libsdl3-image-dev libcurl4-openssl-dev zlib1g-dev imagemagick
-```
+    #   Fedora / RHEL / CentOS
+    sudo dnf install git gcc-c++ cmake libcurl-devel libzip-devel ImageMagick
+    sudo dnf SDL3-devel SDL3_image-devel
 
-#### Fedora / RHEL / CentOS
+    #   Arch Linux
+    sudo pacman -Syu
+    sudo pacman -S base-devel cmake imagemagick libzip libcurl-gnutls zlib
+    sudo sdl3 sdl3_image
 
-```bash
-sudo dnf install git gcc-c++ cmake SDL3-devel SDL3_image-devel libcurl-devel libzip-devel ImageMagick
-```
+Clone the repo with:
 
-#### Arch Linux
+    git clone https://github.com/linappleii/linapple.git
+    cd linapple
 
-```bash
-sudo pacman -Syu
-sudo pacman -S base-devel cmake imagemagick libzip sdl3 sdl3_image libcurl-gnutls zlib
-```
 
-### Clone
+Configure and Compile
+---------------------
 
-```bash
-git clone https://github.com/linappleii/linapple.git
-cd linapple
-```
+    #   Create a build directory and configure with CMake (skipping tests
+    #   for a faster build). For further configuration options, see below.
+    cmake -B build -DBUILD_TESTING=OFF
 
-### Configure and Compile
+    # Compile using all available CPU cores
+    cmake --build build -j$(nproc)
 
-```bash
-# Create a build directory and configure with CMake (skipping tests for a faster build)
-cmake -B build -DBUILD_TESTING=OFF
+#### Configuration Options
 
-# Compile using all available CPU cores
-cmake --build build -j$(nproc)
-```
-
-#### Build Options
 You can pass various options to the `cmake` configuration step:
-- `-DFRONTEND=sdl3` : (Default) Build the emulator with the SDL3-based graphical frontend.
-- `-DFRONTEND=tui` : Build for the terminal using 24-bit color and Unicode characters (no GUI dependencies required).
-- `-DFRONTEND=headless` : Build the emulator without GUI or SDL dependencies (useful for automated testing or server environments).
-- `-DBUILD_TESTING=OFF` : Skip building the test suite (saves compilation time and dependency fetching).
-- `-DREGISTRY_WRITEABLE=ON` : Enable saving emulator configuration settings back to the config file.
+- `-DFRONTEND=sdl3` : (Default) Build the emulator with the SDL3-based
+  graphical frontend.
+- `-DFRONTEND=tui` : Build for the terminal using 24-bit color and Unicode
+  characters (no GUI dependencies required).
+- `-DFRONTEND=headless` : Build the emulator without GUI or SDL
+  dependencies (useful for automated testing or server environments).
+- `-DBUILD_TESTING=OFF` : Skip building the test suite (saves compilation
+  time and dependency fetching).
+- `-DREGISTRY_WRITEABLE=ON` : Enable saving emulator configuration settings
+  back to the config file.
+- `-DFRONTEND=sdl3` : (Default) Build the emulator with the SDL3-based
+  graphical frontend.
+- `-DFRONTEND=headless` : Build the emulator without GUI or SDL
+  dependencies (useful for automated testing or server environments).
+- `-DBUILD_TESTING=OFF` : Skip building the test suite (saves compilation
+  time and dependency fetching).
+- `-DREGISTRY_WRITEABLE=ON` : Enable saving emulator configuration settings
+  back to the config file.
 - `-DPROFILING=ON` : Enable `gprof` profiling output.
-- `-DCMAKE_BUILD_TYPE=Debug` : Build with debugging symbols instead of release optimizations.
+- `-DCMAKE_BUILD_TYPE=Debug` : Build with debugging symbols instead of
+  release optimizations.
 
-### Run Locally
+### Run Locally to Test
 
-After building, you can run the emulator directly from the build output directory:
+After building, you can run the emulator directly from the build output
+directory:
 
-```bash
-cd build
-./linapple
-```
+    cd build
+    ./linapple
 
-Or, to boot automatically into the standard Apple floppy disk provided with LinApple:
+    #   Or to use the standard floppy image provided with LinApple:
+    ./linapple --autoboot --d1 ../res/Master.dsk
 
-```bash
-./linapple --autoboot --d1 ../res/Master.dsk
-```
 
-### Installation
+Installation
+------------
 
 To install LinApple so it can be run from anywhere, use the `install` target.
 
-```bash
-cmake --install build
-```
+    cmake --install build
 
 #### XDG Compliance (Linux)
-The build system uses standard `GNUInstallDirs` and automatically adapts to your privileges:
-- **Non-root install (Default):** If you run `cmake --install build` as a regular user without overriding the prefix, it will install to `~/.local/bin/`, `~/.local/share/linapple/`, and `~/.config/linapple/`. This is fully compliant with XDG Base Directory specifications and does not require `sudo`.
-- **System-wide install (Root):** If you run `sudo cmake --install build`, it will install system-wide to `/usr/local/bin/`, `/usr/local/share/linapple/`, and `/usr/local/etc/linapple/`.
+
+The build system uses standard `GNUInstallDirs` and automatically adapts to
+your privileges:
+- __Non-root install (Default):__ If you run `cmake --install build` as a
+  regular user without overriding the prefix, it will install to
+  `~/.local/bin/`, `~/.local/share/linapple/`, and `~/.config/linapple/`.
+  This is fully compliant with XDG Base Directory specifications and does
+  not require `sudo`.
+- __System-wide install (Root):__ If you run `sudo cmake --install build`,
+  it will install system-wide to `/usr/local/bin/`,
+  `/usr/local/share/linapple/`, and `/usr/local/etc/linapple/`.
 
 You can also explicitly define your installation prefix during configuration:
-```bash
-cmake -B build -DCMAKE_INSTALL_PREFIX=/usr
-sudo cmake --install build
-```
+
+    cmake -B build -DCMAKE_INSTALL_PREFIX=/usr
+    sudo cmake --install build
 
 After installation, simply run:
-```bash
-linapple
-```
+
+    linapple
 
 ### Configuration and Assets
 
-LinApple expects to find its configuration file (`linapple.conf`) in your configuration directory (e.g., `~/.config/linapple/linapple.conf`).
+LinApple expects to find its configuration file (`linapple.conf`) in your
+configuration directory (e.g., `~/.config/linapple/linapple.conf`).
 
-If the emulator cannot find a required asset (like `Master.dsk` or character fonts) in the current directory, it will automatically search the `share` and `config` directories established during installation.
+If the emulator cannot find a required asset (like `Master.dsk` or
+character fonts) in the current directory, it will automatically search the
+`share` and `config` directories established during installation.
diff --git a/Test b/Test
new file mode 100755
index 0000000..2d4b549
--- /dev/null
+++ b/Test
@@ -0,0 +1,135 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+trap 'ec=$?; echo 1>&2 "INTERNAL ERROR: ec=$ec line=$LINENO cmd=$BASH_COMMAND";
+    exit $ec;' ERR
+
+####################################################################
+
+error() { echo -e 1>&2 "● ERROR: $(basename "$0"):" "$@"; }
+die()   { local ec=$1; shift; error "$@"; exit $ec; }
+
+#   Given $1 as a {distro}-{release}-{frontend} string,
+#   set globals $distro $release $frontend to its components,
+#   and $platform to colon-separated $distro:$release.
+#   These are implicit parameters to many routines.
+parse_build_spec() {
+    declare -g platform distro release frontend
+    local spec="$1"; shift
+    [[ $spec =~ ^([^-]+)-([^-]+)-([^-]+)$ ]] || die 2 "Bad build spec: $spec"
+      distro="${BASH_REMATCH[1]}"
+     release="${BASH_REMATCH[2]}"
+    frontend="${BASH_REMATCH[3]}"
+    platform="$distro:$release"
+}
+
+docker_unavailable() {
+    die 3 'Cannot run $docker due to setup_docker() failure (see above).'
+}
+
+setup_docker() {
+    declare -g docker=docker
+    if ! $docker --version >/dev/null 2>&1; then
+        echo 1>&2 "WARNING: Cannot run '$docker' command. Check path?"
+        docker=docker_unavailable
+    elif ! $docker info >/dev/null 2>&1; then
+        docker='sudo docker'
+        if ! sudo -v 2>/dev/null; then
+            echo 1>&2 "WARNING: Cannot sudo to run '$docker'; start proxy?"
+            docker=docker_unavailable
+        fi
+    fi
+}
+
+####################################################################
+
+build_image() {
+    echo "───── Build image $platform"
+    #   XXX Actually, it would be nice to use a single image for all
+    #   frontends, but we'd have to figure out how we pre-install all
+    #   the frontend deps, rather than just one.
+    declare -g image_name="linapple-build" \
+                image_tag="$distro-$release-$frontend"
+    $docker build \
+        --tag "$image_name:$image_tag" \
+        --build-arg PLATFORM="$platform" \
+        --build-arg FRONTEND="$frontend" \
+        "$PROJDIR/btools"
+}
+
+run_container() {
+    echo "───── Run container"
+    #   --chown-to leaves the bind-mounted build dir owned by the host user.
+    $docker run --rm \
+            --volume "$PROJDIR:$PROJDIR" \
+            --workdir "$PROJDIR" \
+        "$image_name:$image_tag" \
+        btools/build --chown-to "$(id -u):$(id -g)" "$frontend"
+}
+
+####################################################################
+#   Usage and argparse.
+
+#   Default list if no distros given as $@.
+#   XXX but how do we provide non-default SDLs?
+default_builds=(
+    debian-13-sdl3 debian-12-sdl2 debian-12-sdl1.2
+   #debian:12  debian:13  ubuntu:24.04  ubuntu:26.04
+   #fedora:38  fedora::43
+   #arch:latest
+)
+
+usage() {
+    local exitcode=$1; shift
+    [[ -n ${1:-} ]] && error "$@"
+    cat <<_____
+Usage: $(basename "$0") --current [BUILD-ARGS]
+       $(basename "$0") [OPTS] [BUILD-SPEC...]
+
+Calls btools/build for one or more Linux build specifications, which are in
+{distro}-{release}-{frontend} format, e.g. 'ubuntu-24.04-sdl2'. All
+builds (whether local or in containers) are placed under
+'build/{distro}-{release}-{frontend}/'.
+
+With --current, does a build for the current host rather than using
+containers. BUILD-ARGS are passed to btools/build. (Run 'btools/build -h'
+for further information.)
+
+Otherwise starts a container for each BUILD-SPEC and does the build and
+unit tests in that container, leaving the results under build/ on the host.
+
+The default build specs are: ${default_builds[@]}.
+
+_____
+    exit $exitcode
+}
+
+build_current=false
+while [[ $# -gt 0 ]]; do case "$1" in
+    -h|--help)      usage 0;;
+    --current)      shift; build_current=true;;
+    -*)             usage 2 "Unknown option: $1";;
+    --)             break;;
+    *)              break;;
+esac; done
+
+####################################################################
+#   Main
+
+PROJDIR=$(command cd $(dirname "$0") && pwd -P)
+
+$build_current && {
+    "$PROJDIR/btools/build" "$@"
+    exit 0
+}
+
+setup_docker
+for build_spec in "${@:-${default_builds[@]}}"; do
+    echo "━━━━━ $build_spec"
+    parse_build_spec "$build_spec"  # sets $distro $release $frontend $platform
+    build_image
+    run_container
+done
+
+#   XXX --current vs. --all ?
+#   XXX -d DISTRO -f FRONTEND   (either implies --current?)
+#               no, maybe better limits --all? patterns? `-d deb` for deb*?
diff --git a/btools/Dockerfile b/btools/Dockerfile
new file mode 100644
index 0000000..c407d57
--- /dev/null
+++ b/btools/Dockerfile
@@ -0,0 +1,9 @@
+# check=skip=InvalidDefaultArgInFrom   $PLATFORM unset produces good error
+ARG PLATFORM
+FROM ${PLATFORM}
+ARG FRONTEND PLATFORM
+#   Cache the installed dependencies in the image so that, at least for
+#   a while, the build based on this image will find all prerequisites
+#   already installed, speeding the build.
+COPY build /tmp/
+RUN /tmp/build --prereq-only ${FRONTEND} && rm /tmp/build
diff --git a/btools/build b/btools/build
new file mode 100755
index 0000000..893ed3b
--- /dev/null
+++ b/btools/build
@@ -0,0 +1,179 @@
+#!/usr/bin/env bash
+#
+#   btools/build - Build linapple for a distro/frontend
+#
+#   See usage() for details.
+#
+set -Eeuo pipefail
+trap 'ec=$?; echo 1>&2 "INTERNAL ERROR: ec=$ec line=$LINENO cmd=$BASH_COMMAND";
+    exit $ec;' ERR
+
+error() { echo -e 1>&2 "● ERROR: $(basename "$0"):" "$@"; }
+die()   { local ec=$1; shift; error "$@"; exit $ec; }
+
+#   $1 is a timestamp dir/file. If modified within the last hour,
+#   return 1. Otherwise touch it and return 0.
+needs_update() {
+    local timestamp="$1"; shift
+    [[ -e $timestamp ]] || { touch "$timestamp"; return 0; }
+    local age=$(( $(date +%s) - $(stat -c %Y "$timestamp") ))
+    (( age <= ${1:-3600} )) || { touch "$timestamp"; return 0; }
+    return 1
+}
+
+sudo=
+set_sudo() {
+    declare -g sudo
+    [[ $(id -u) -eq 0 ]] || sudo=sudo
+}
+
+chown_builddir() {          # Used by --chown-to.
+    local to="$1"; shift
+    chown -R "$to" "$PROJDIR/$BUILDDIR_REL" \
+        || echo 1>&2 "WARNING: chown -R '$to' '$BUILDDIR_REL' failed"
+}
+
+####################################################################
+
+#   Set variables defined in /etc/os-release, in particular $ID and
+#   $VERSION_ID. If /etc/os-release doesn't exist these may be synthesised
+#   through other means, otherwise this exits with an error.
+os_release() {
+    [[ -r /etc/os-release ]] \
+        || die 1 "/etc/os-release not found; cannot autodetect distro"
+    source /etc/os-release
+    [[ -n ${ID:-} && -n ${VERSION_ID:-} ]] \
+        || die 5 '$ID and/or $VERSION_ID not set in /etc/os-release'
+}
+
+#   We know os_release but no FRONTEND specified: choose a default or
+#   exit with an error indicating we have no default.
+default_frontend() {
+    local rel=$VERSION_ID
+    case $ID in
+        debian)     [[ $rel -ge 13 ]] && echo sdl3 || echo sdl2;;
+        ubuntu)     [[ $rel  >  24.04 ]] && echo sdl3 || echo sdl2;;
+        *)          die 4 "No default frontends known for distro $ID";;
+    esac
+}
+
+install_prerequisites() {
+    echo '━━━━━ Install prerequisites'
+
+    #   This needs to be on a filesystem local to the container, if we're using
+    #   one, so that we don't honour a stale timestamp in a new container.
+    local tsfile="/tmp/linappleii-linapple/prereq-$ID-$VERSION_ID-$FRONTEND"
+    mkdir -p "$(dirname "$tsfile")"
+    #   If we've updated and re-installed in the last hour, just skip.
+    needs_update "$tsfile" \
+        || { echo 'updated in last hour; skipping'; return 0; }
+
+    local known_ids=(       # all these must be in the `case` below.
+        debian ubuntu
+    )
+    local id k
+    for distro in "$ID" ${ID_LIKE:-}; do        # split $ID_LIKE
+        for k in "${known_ids[@]}"; do
+            #   Stop both loops at first one we know.
+            [[ $distro == "$k" ]] && break 2
+        done
+        die 4 "This system's IDs match no known distros" \
+            "\n• Known:" "${known_ids[@]}" \
+            "\n• ID=$ID ID_LIKE=${ID_LIKE:-}"
+    done
+
+    case "$distro" in
+        debian|ubuntu)  install_deps_debian;;
+        *)              die 10 "Internal error: set unknown distro '$distro'";;
+    esac
+}
+
+install_deps_debian() {
+    echo '───── apt update'
+    $sudo apt update && $sudo touch /var/lib/apt/lists
+    echo '───── apt install'
+    $sudo apt install -y -q \
+        git g++ cmake \
+        libzip-dev libcurl4-openssl-dev zlib1g-dev imagemagick
+    case "$FRONTEND" in
+        sdl1.2) $sudo apt install -y -q libsdl1.2-dev;;
+        sdl2)   $sudo apt install -y -q libsdl2-dev libsdl2-image-dev;;
+        sdl3)   $sudo apt install -y -q libsdl3-dev libsdl3-image-dev;;
+        *)      die 4 "Unknown frontend: $FRONTEND";;
+    esac
+}
+
+####################################################################
+#   Usage and argument parsing.
+
+usage() {
+    local exitcode=$1; shift
+    [[ -n ${1:-} ]] && error "$@"
+    cat <<_____
+Usage: $(basename "$0") [OPTS] [-D DISTRO] [FRONTEND]
+For the current distro  and given (or default) frontend, installs
+prerequisites, builds linapple and runs tests.
+
+• The distro is auto-detected from /etc/os-release, but can be overridden with
+  e.g. '-D ubuntu:24.04' (but you should never need this).
+Options:
+  -h,--help         Print this message.
+  -D ID:VERSION_ID  Use given distro information instead of autodetecting
+                    from /etc/os-release. E.g. 'ubuntu:24.04'.
+  --prereq-only     Only install build prerequisites; do not build.
+  --chown OWNER     On exit, chown -R the build dir to OWNER (UID:GID or
+                    user:group). For container builds writing to a bind-
+                    mounted host dir; failures warn but do not abort.
+_____
+    exit $exitcode
+}
+
+chown_to=
+force_distro=
+prereq_only=false
+FRONTEND=
+while [[ $# -gt 0 ]]; do case "$1" in
+    -h|--help)      usage 0;;
+    -D)             shift; force_distro="$1"; shift;;
+    --prereq-only)  shift; prereq_only=true;;
+    --chown-to)     shift; chown_to="$1"; shift;;
+    -*)             usage 2 "Unknown option: $1";;
+    --)             break;;
+    *)              break;;
+esac; done
+[[ $# -gt 0 ]] && { FRONTEND="$1"; shift; }
+[[ $# -eq 0 ]] || usage 2 "Too many arguments."
+
+if [[ -z $force_distro ]]; then
+    os_release
+else
+    if [[ $force_distro =~ ^([^:]+):([^:]+)$ ]]; then
+        ID=${BASH_REMATCH[1]}
+        VERSION_ID=${BASH_REMATCH[2]}
+    else
+        die 2 "DISTRO must be 'name:ver'"
+    fi
+fi
+
+[[ -n $FRONTEND ]] || FRONTEND=$(default_frontend)
+
+####################################################################
+
+PROJDIR=$(command cd $(dirname "$0")/.. && pwd -P)
+#   No colon in builddir path because even on Linux it makes Cmake upset.
+#   This is relative because we need to `cd` to `cmake` anyway, so why
+#   not keep it short for messages etc.
+echo "▶ DISTRO=$ID:$VERSION_ID FRONTEND=$FRONTEND"
+
+install_prerequisites
+$prereq_only && exit 0
+
+command cd "$PROJDIR"
+echo '━━━━━ Cmake setup'
+BUILDDIR_REL="build/$ID$VERSION_ID-$FRONTEND"
+echo "▶ BUILDDIR=$BUILDDIR_REL"
+[[ -n $chown_to ]] && trap "chown_builddir $chown_to" EXIT
+cmake -B "$BUILDDIR_REL" -DFRONTEND=$FRONTEND
+echo '━━━━━ Cmake build'
+echo ● cmake --quiet --build "$BUILDDIR_REL" -j$(nproc)
+cmake --build "$BUILDDIR_REL" -j$(nproc)