Optimise Docker image size

[paulhowardarm]

Summary

Make the resulting Docker image as small as possible without impacting its reliability and portability.

Details

The Dockerfile intentionally uses multi-stage builds to avoid having unwanted toolchain components or partial build products in the final runtime image. The runtime image is currently based on ubuntu:20.04, which works, but results in ~100Mb final image size. Is there a smaller base image we can use without compromising functionality? Could we start with Alpine and ensure that glibc and bash are added, for instance?