Docker security - support secrets

It would be helpful for the docker image and compose example to properly support Docker secrets. Generally this is achieved by adding an env var NNNN_FILE where NNNN is any env var that should contain sensitive info. The NNNN_FILE will then be pointed to a mounted secret containing the sensitive data and the docker container/software will need to read that file rather than the NNNN env var.

See postgres' own docker container and POSTGRES_PASSWORD_FILE as an example of implementation.