There is a convention on official docker images that environment variables that end in
_FILE are read as a file and then assigned to a variable without the
_FILE suffix. For example, setting the environment variable
/run/secrets/secret-password will read the file
/run/secrets/secret-password and assign its contents to the environment variable
ELASTIC_PASSWORD. This is especially handy when using Docker secrets.
Related issue on grafana: https://github.com/grafana/grafana-docker/pull/166