Suggested Dockerfile improvements

Improvements:

• separate out go mod download to utilise build cache.
• explicit in source repo for golang builder image (other container runtimes may not default to docker.io).
• switch to UID 1000 to help reduce privileges of the running binary in the container to reduce potential container escape attack surface.
• -w -s in the go build step to strip out debugging symbols to reduce binary size.
• default to main as VERSION.
• shallow git clone with --depth=1 since we don't need a full git history to perform a build.
• golang 1.17 rather than 1.15, some improvements in runtime performance and binary size.