Set ENVIRONMENT as env var to service container

[lorenyu]

I discovered that the flask template relies on an environment variable called ENVIRONMENT being set (e.g. ENVIRONMENT="dev", ENVIRONMENT="prod"). While not an ideal dependency, we should still set it anyways from infra point of view.

[lorenyu]

higher priority since flask template currently relies on this

[lorenyu]

actually on second thought i think we should probably prioritize navapbc/template-application-flask#136 and deprioritize this ticket. i'm actually still on the fence as to whether we should do this at all.

[lorenyu]

don't think we should do this unless there's a real need for it. in general want to discourage from using the environment as part of application logic, to stay true to 12factorapp principles