an env override for docker_build #3894

A common idiom in container runners is to have an easy way to override the environment variables, e.g.:

docker run -e
https://docs.docker.com/engine/reference/run/#env-environment-variables

podman run -e
http://docs.podman.io/en/latest/markdown/podman-run.1.html

docker-compose env and envfile

It might make sense for docker_build to have a similar env= argument, similar to how we allow you to override the entrypoint with docker_build(entrypoint)

Related issue: #2643