Added support for docker secrets

[DennisGaida]

Right now the bouncer API key needs to be hardcoded in the compose file and/or in the external .env. The current best practice is to leverage docker secrets that are not only supported in swarm mode, but also from docker compose.

I have added a basic implementation within the RequiredEnv function, that allows the use of <environment_variable_name>+_FILE instead of <environment_variable_name>. So CROWDSEC_BOUNCER_API_KEY becomes CROWDSEC_BOUNCER_API_KEY_FILE. If said [...]_FILE variable exists the value is read from the file and returned from the RequiredEnv function.

I'd greatly appreciate some testing since logging from config.go seems kind of impossible? Even the existing log.Fatalf() entries I never get to see in the docker logs (though the container doesn't start).

Example docker-compose:

version: "3.8"

secrets:
  crowdsec_bouncer_api_key:
    file: /secretsPath/crowdsec_bouncer_api_key

services:
  bouncer:
    build: .
    image: fbonalair/traefik-crowdsec-bouncer
    container_name: bouncer
    environment:
      CROWDSEC_BOUNCER_API_KEY_FILE: /run/secrets/crowdsec_bouncer_api_key
      CROWDSEC_AGENT_HOST: crowdsec:8123
    secrets:
      - crowdsec_bouncer_api_key

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