docker.build should invalidate results when environment changes

[lolgab]

Docker images are built when the environment changes, since Docker depends on some environment variables.

Docker documentation here.

Mill should invalidate the docker build target when the environment changes.

[lefou]

Since the DockerModule already defines a envVars target, I'd argue that the current implementation assumes they are the only used env vars at play. This is probably not correct, since os-lib's os.proc is silently using all default env vars of the Java process if the propagateEnv parameter isn't set to false. So a fix should probably set it. If a user wants to depend on all actual env vars, she already can override envVars with T.input { sys.props }. Or we implement it that way by default.