Resolve "Multiple configs presets"

Closes #676 (closed)

Implements !590 (comment 363729) with option 1:

There is a config/esrf and a config/local directory and an env variable to say at build time wich one to put as default config in the image.

This means we'll still need to commit config updates here and build/release new versions for config changes.

It's not ideal but it allows flexibility since we can create config for any other preset we would need and switch at build time. Replacing the config at run time with docker volumes is still available but we need to see if this is something we can use with portainer or not.

Documentation was updated to reflect that.