Spotify Connect: make the advertised host ip an explicit setting

[magedhennawy]

from the #354 thread on api/spotify.py:182.

right now the advertised address is worked out by inspecting local interfaces, which doesn't survive docker. the container's own ip is what gets picked up, and a spotify client on the lan can't reach that. auto detection tricks don't help either, opening a socket toward an external address just returns whichever interface the container would route out of, so it has the same problem.

proposal from the thread: make it an explicit setting.

• config entry for the host ip, with the existing env var as a fallback
• a field in the frontend settings so it's discoverable
• when not containerised, default it to the detected value so a bare metal install still works with no setup
• leave it blank in docker rather than guessing, so it's obvious it needs setting instead of silently advertising something unreachable
• log or surface a warning when nothing usable is configured

worth knowing there are two separate reports of the device never showing up, #316 and #304, both macos. those may be a different cause, librespot advertises via gethostbyname(gethostname()) which on macos often resolves to 127.0.0.1, but an explicit host ip setting would give those users a way out regardless.

[moddroid94]

I changed the logic of the spotify.py (L175) module to load a config variable named DOCKER_ETH_ADDRESS

so we just need to add this to the default config as empty, and then check on service start, and throw an error if no config is set.

for now it has fallback to 8.8.8.8 returned route only.

i won't suggest supporting bare metal no-config setup, that may not work and create more issue or confusion, just explain how to set the ip for the various installation methods so we can change docs instead of code, it's probably just a copy paste command for most platforms while for us it's quite more code than that lol