Support CONFIG_FILE_PATH for proxy (Easier Azure container deployment)

[krrishdholakia]

          Having the CONFIG_FILE_PATH env var enabled allows the app service to point to the image and specify the path to the config in the app service settings. This removes the need to use a docker compose file to pass the --config arg to the container during startup. 

Now that I understand the pattern, the current Docker compose solution isn't bad, it's just not intuitive. Adding some documentation would probably solve the issue for most people.

So having CONFIG_FILE_PATH would be more intuitive, but may not be worth the effort if it requires much work on your end to add it back in. If that is the case, I will happily document what is working for me in Azure.

Originally posted by @lackeyi in #971 (comment)