Add Docker Secrets Support

[GAS85]

Documentation Update
Add more examples to the compose

Now any Variable that set with _FILE at the end will be treated as Docker Secret.

[vogler]

What's the benefit of this over putting your sensitive data in data/config.env (loaded first by dotenv) and everything non-sensitive in the docker-compose.yml (overwrites via env)?

[vogler]

Don't want another place to keep options in sync besides config.js and README.md.
Is this useful for some software to offer a template? (There's already some external Unraid template.)

[GAS85]

That is a good wish. I can remove them, but at least compose example should be copy-paste working, so that it contains basic needed data.

[GAS85]

What's the benefit of this over putting your sensitive data in data/config.env (loaded first by dotenv) and everything non-sensitive in the docker-compose.yml (overwrites via env)?

There is no clear benefit if I read documentation carefully before to do it 😵‍💫. I didn't know that this was supported, sorry for that.

Probably that was caused by my own setup, I do share some Secrets across containers with docker secrets feature, so that I simply need to declare them additionally in a yaml file.

1. E.g. in this case if I have multiple fgc's containers I can setup only once VNC password and do no copy it into the env file across different data directories.
2. I bound few containers together (e.g. own dedicated network via network_mode and depends_on), in this case sometimes docker health checks should be performed in a way that you do not call local container service, but calling local service via other container route and you could need's credentials to authorize yourself in this case. Those credentials are shared as secrets over containers.

To be honest I do not like docker secrets implementation, rather as it is done in kubernetes - you declare secret, it is variable, not a file, that you need to read and import.

[GAS85]

I thought also to limit those to only credentials and add here whitelist, e.g. PASSWORD_FILE, EMAIL_FILE, OTP_FILE.

grep -E "PASSWORD_FILE|EMAIL_FILE|OTP_FILE"