APP_KEY_FILE for docker secrets and passphrases

[5andr0]

Would be nice to have APP_KEY_FILE to use it with docker secrets

Alternatively it would also be nice to have APP_PASSPHRASE(_FILE) and just hash it into the required bit size, instead of forcing the user to use 32 chars.

In case of an APP_KEY loss it's easier for the user to remember the passphrase.

Thanks for the project!

[Bubka]

Indeed, a passphrase could be easiest to remember, although whatever the secret format, it should be backed up.
Having multiple options to achieve the same thing is to me the best approach to ease adoption, I will add it.