Fix #19: add alpine image

alpine/Dockerfile

@@ -0,0 +1,20 @@
+FROM alpine:3.10
+
+ENV RUSTUP_HOME=/usr/local/rustup \
+    CARGO_HOME=/usr/local/cargo \
+    PATH=/usr/local/cargo/bin:$PATH
+
+RUN set -eux; \
+    apk add --no-cache \
+    ca-certificates \
+    gcc; \
+    \
+    url="https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-musl/rustup-init"; \
+    wget "$url"; \
+    chmod +x rustup-init; \
+    ./rustup-init -y --no-modify-path --default-toolchain nightly; \
+    rm rustup-init; \
+    chmod -R a+w $RUSTUP_HOME $CARGO_HOME; \
+    rustup --version; \
+    cargo --version; \
+    rustc --version;