Build with any environment, defaulting to prod

[cstar]

This PR hopefully works. I use a docker MIX_ENV and release env, different from the prod env.

[teamon]

If I understand correctly, this is a breaking change since running $ mix docker.build in dev will build a dev release instead of prod, right?

[cstar]

No, unless you specify explicitely the environment it will behave like your current code I.e running in prod env