[FEATURE] Allow API_KEY vars to be set from docker secret paths

[Flaminel]

Discussed in #106

^{Originally posted by a3gill March 28, 2025}
Pretty straight-forward. Allow setting API Keys with Docker secrets.
Either allow any VARNAME to be set from the contents of a file path assigned to FILE__VARNAME, or create optional vars for each API KEY var named VARNAME_FILE that accepts a file path that will result in setting VARNAME to the cat result of the supplied path.

Thanks!