Adding support for docker secrets

[kitingChris]

To pass the value for --requirepass password in a secure way docker secrets can be used with this pr.

If a secrets file is available and not empty it the password is passed to the server option --requirepass

[tianon]

Duplicate of https://github.com/docker-library/redis/issues/268

(see especially https://github.com/docker-library/redis/issues/268#issuecomment-776877231)

[kitingChris]

This is not an issue but an pull request. It solves #268. While the ACL approach may be the better solution my goal only was to get it running with requirepass and passing the password as secret.

[tianon]

When we said we didn't plan to implement it, we also meant we didn't plan to accept/maintain the code (sorry for the lack of explicit clarity there).

[kitingChris]

And why is it not planned to implement it? It is not a very complex code.

EDIT: the requirepass functionality is in many scenarios still the most used one. In my case with nextcloud. Using an ACL would mean to manage the password in 2 separate ways instead of one.