Add support for Docker secrets

[mathieupotier]

Add support for docker secrets : https://docs.docker.com/engine/swarm/secrets/

Created new environment var to manage secrets source files.

Hope it helps, made such a change for my usage, and seems working perfectly ^_^

[J0WI]

I'd prefer to use the same pattern as other php images do. Would you like to implement this?

[mathieupotier]

I can do it, I'll push this in few hours.

[mathieupotier]

Done.

I just prefered to not export env var to avoid leaks on secrets. The replaced env vars are just shared under current PID, they don't go to the kernel stack.