WebUI: Make WebUI docker- and reverse proxy- friendly

[sgofferj]

Currently, the WebUI doesn't seem to be very rproxy friendly. It needs to be told the IP for the API(?) in the environment variable IP. The UI then only connects back to this IP. As a consequence, the UI cannot be multihomed and it cannot be accessed through a reverse proxy. That makes life quite hard for admins who like to run FTS in a secure way in a docker environment behind a reverse proxy, even more so if it should handle SSL encryption.

[RadioAndrea]

The current setup listens to the catch-all address, as far as I can tell.

Additional ip is no longer hard coded, and can be set in the config file.

New docker fixes allow working with the config file.