Add build arguments for docker builds

[nocode99]

From a quick convo in the gitter channel, I think a nice addition to mu would be having a section for docker build arguments in mu.yml. Some nodejs apps use environment variables to generate the static files and there's currently no way to do that with mu

Example: docker build --build-arg ENV=prod -t my-app .

# mu.yml
......
build_args:
  - ENV=prod