Add support for reading config parameters from files for use with docker secrets about traefik-forward-auth

[thomseddon]

Hi,

This can already be done (it's how I pass those secrets :), it's described at the end of the configuration section of the README: https://github.com/thomseddon/traefik-forward-auth/blob/master/README.md#configuration

Let me know if you have any problems

[maximunited]

I don't think this is working in v2. at least not for me:
I've tried:

PROVIDERS_GOOGLE_CLIENT_ID_FILE=/run/secrets/google_client_id
PROVIDERS_GOOGLE_CLIENT_ID=/run/secrets/google_client_id
CLIENT_ID_FILE=/run/secrets/google_client_id
CLIENT_ID=/run/secrets/google_client_id

[maximunited]

I don't think this is working in v2. at least not for me:
I've tried:

PROVIDERS_GOOGLE_CLIENT_ID_FILE=/run/secrets/google_client_id
PROVIDERS_GOOGLE_CLIENT_ID=/run/secrets/google_client_id
CLIENT_ID_FILE=/run/secrets/google_client_id
CLIENT_ID=/run/secrets/google_client_id

answering to myself. The answer was in issue #147
configuring docker-compose works:

    environment:
      CONFIG: /run/secrets/credentials
    secrets:
      - credentials

secrets:
  credentials:
    name: oauth2_credentials
    file: ${SECRETSDIR}/oauth2_credentials.ini