[Feature Request] Support _FILE suffix in docker images

[nikeee]

Feature Description

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 ELASTIC_PASSWORD_FILE to /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: grafana/grafana-docker#166
Thread on forums: https://discuss.elastic.co/t/feature-request-support-file-suffix-in-docker-images/140107

[rjernst]

Maintenance of the docker files for elasticsearch has moved to the elasticsearch repo. As we will be archiving this repository, I am going to close this issue and I have migrated it to elastic/elasticsearch#43603.