Add "file_env" support, especially for Docker secrets

[tianon]

This adds explicit support for the following:

• MYSQL_ROOT_PASSWORD_FILE
• MYSQL_DATABASE_FILE
• MYSQL_USER_FILE
• MYSQL_PASSWORD_FILE

See also:

• Add support for passing a file path as the value for DB password wordpress#186
• Add "file_env" support, especially for Docker secrets postgres#225
• Add "file_env" support, especially for Docker secrets redmine#43

I've only updated 8.0/docker-entrypoint.sh here so that this can serve as a straw-man for discussion -- once the discussion concludes, I'll update the PR with the result and push the functionality across the board to all supported versions. 👍

[tianon]

I saw this was added to mysql/mysql-server in mysql@0473555, but IMO it's cleaner to add an explicit "get the password from this specific file" option than to overload the existing option. For example, if someone uses a bad -w/--workdir on their container, their password might be accidentally interpreted as a file to read, and there will be no external indication that it's happened and the admin will be scratching their head wondering why their password isn't working. 😅

(That being said, we defer to you, @ltangvald -- however you think this should be implemented here is the direction we'll go. 👍)

[tianon]

cc @cyli @mstanleyjones FYI

[ltangvald]

Hm, yeah. I'm not 100% sure why we used the same variable (other than a general desire to keep the number of extra env variables down). We considered the chance of accidentally setting the password to a valid file path low, but I agree it's cleaner to make it explicit.
The file_env function looks a bit complex to me, but it's nice to have the functionality and I can't think of a simpler way off the top of my head, so overall I think this looks good :)

[tianon]

Yeah, the goal of the function was to abstract the behavior in a clean, generic way so we don't have "this or that" logic peppered all over the file (which is error prone on top of verbose). 😅

I've updated to push this change to all the versions now. 😄 👍