Add base image configuration info to windows/README.md

Author: ecstatic-morse

windows/README.md

@@ -1,9 +1,8 @@
 # Windows build environment for rust crates
 
 This repository contains the source of a Docker container the Rust project uses
-to build third-party crates on Windows. It is based on **Windows Server Core,
-version 1803**, and contains all the native dependencies used by the Rust
-crates we know of.
+to build third-party crates on Windows. It is based on **Windows Server Core**,
+and contains all the native dependencies used by the Rust crates we know of.
 
 ## Dependencies
 
@@ -16,6 +15,10 @@ This image must be run on a Windows host with both containerization and Hyper-V
 enabled (Windows 10 Pro or Windows Server >=2016). If you wish to run `crater`
 on an Azure VM, this requires a virtual machine image tagged with "v3".
 
+The version of the base image can be configured by passing `--build-arg
+BASE_IMAGE_VER=${YOUR_DESIRED_VERSION_NUMBER}` to `docker build`. It defaults
+to `:1803` (note the leading colon) which specifies Windows Server Core,
+version 1803.
 
 Hyper-V can be enabled in [the GUI][hyperv] or in `Powershell` with:
 
@@ -29,7 +32,7 @@ A reboot is required for this to take effect.
 
 
 The Docker host must have a [build number][build] equal to that of the image
-you wish to run (presently `mcr.microsoft.com/windows/servercore:1803`).
+you wish to run (by default `mcr.microsoft.com/windows/servercore:1803`).
 
 [Docker Desktop][] is required to run native Windows containers; this image
 will not work with Docker Toolbox. Once installed, ensure that Docker Desktop