Dockerfile: Use an argument for selecting the build image.

Dockerfile

@@ -1,5 +1,8 @@
+# -- stage 0: some configuration
+ARG BASE_IMG=alpine:3.13.5
+
 # -- stage 1: build static routinator with musl libc for alpine
-FROM alpine:3.13.5 as build
+FROM ${BASE_IMG} as build
 
 RUN apk add rust cargo
 
@@ -13,7 +16,7 @@ RUN cargo build \
 
 # -- stage 2: create alpine-based container with the static routinator
 #             executable
-FROM alpine:3.13.5
+FROM ${BASE_IMG}
 COPY --from=build /tmp/routinator/target/x86_64-alpine-linux-musl/release/routinator /usr/local/bin/
 
 # Build variables for uid and guid of user to run container