Support container secrets

[rockaut]

It would be great if Zabbix could also support using container secrets. Currently the common implementation is to look at _FILE environment variables and if there are some use that provided file for reading the environment variable.

For example with POSTGRES_PASSWORD:
If the env variable POSTGRES_PASSWORD_FILE is set read the contents of the POSTGRES_PASSWORD_FILE into POSTGRES_PASSWORD environment variable (even overwriting already set POSTGRES_PASSWORD).

Also look at implementation in grafana for example: https://grafana.com/docs/installation/docker/#reading-secrets-from-files-support-for-docker-secrets and grafana/grafana-docker#166
By the way - Postgres is already supporting this in their containers!

[dotneft]

Added