Set a custom IP via env variable

[muchtall]

Since moving to using docker images to run Octoprint, Detailed Progress now reports the container IP on the printer display. I'm aware that this display option can be disabled entirely. I was thinking it would be nice to override the IP discovery logic in Detailed Progress with a "static" value that could be set via environment variable, like $HOST_IP which could be set in the docker container variables.

[muchtall]

Apologies my PR doesn't have documentation changes. I was going to update it, but then didn't know where exactly it should go since the existing "Show IP at startup" setting doesn't seem to have a documented yaml config file flag.