Added some code in all entrypoints to support docker secrets (https://docs.docker.com/engine/swarm/secrets/) as requested here: monicahq/monica#5809

Instead of supporting secrets (chosen naming convention is <environment variable name> + _FILE suffix, e.g. DB_PASSWORD_FILE - some containers use __FILE) for all environment variables, a set of supported secrets is defined in supportedSecrets. Then it is checked whether the environment variable with the suffix exists and whether the secrets file exists.

If all checks pass, the original environment variable is overwritten with the contents contained in the secrets file.

It can be tested with the following example docker-compose.yml:

secrets:
  monicahq_db_password:
    file: $SECRETSDIR/monicahq_db_password

services:
  monicahq:
    image: monica
    container_name: monica
    environment:
      - DB_PASSWORD_FILE=/run/secrets/monicahq_db_pasword
    secrets:
      - monicahq_db_password

I haven't updated the docker-compose.yml examples as I first want to check the waters.