nginx: add /websockets plural alias with path rewrite to selkies /ws

[DL6ER]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Add an nginx location block for the plural /websockets URL that the selkies-dashboard frontend builds in WebSocket mode, with a path rewrite to selkies' /ws endpoint.

location SUBFOLDERwebsockets {
    proxy_set_header        Upgrade $http_upgrade;
    proxy_set_header        Connection "upgrade";
    ...
    proxy_pass              http://127.0.0.1:CWS/ws;
}

Done in both the :3000 and the :3001 server blocks of root/defaults/default.conf.

Benefits of this PR and context:

Out of the box, opening a freshly started lscr.io/linuxserver/baseimage-selkies-derived container in a browser logs:

Firefox can't establish a connection to the server at wss://<host>/websockets.
[websockets] Error: WebSocket connection refused

Three independent path conventions are in play:

• The frontend (addons/selkies-web-core/selkies-ws-core.js upstream) builds the URL as ${ws_protocol}${window.location.host}${pathname}websockets (plural).
• The bundled nginx config in this repo only declares location SUBFOLDERwebsocket {} (singular).
• The selkies signaling server in src/selkies/signaling_server.py upgrades to WebSocket only for path == "/ws" or paths ending in /signaling.

So even a singular /websocket request would 404 inside selkies.

The proposed change makes the default install work end-to-end without patching either the frontend or selkies, by exposing the URL the frontend already uses and rewriting it to the path the server expects. The existing singular /websocket location is left untouched for backwards compatibility with anything that may already point at it.

How Has This Been Tested?

Built locally from this branch on top of the upstream master (debian-trixie variant), then composed a webtop-style layer on top (debian-xfce) and ran it on a Linux x86_64 host with:

docker run -d \
  -p 3010:3000 \
  -e PUID=1000 -e PGID=1000 -e TZ=Europe/Berlin \
  -e SELKIES_TURN_HOST=... \
  -e SELKIES_TURN_PORT=3478 \
  -e SELKIES_TURN_PROTOCOL=udp \
  -e SELKIES_TURN_SHARED_SECRET=... \
  --shm-size=1g --cap-add SYS_ADMIN \
  webtop:patched

Validated in Firefox 149 and Chromium 142 on Ubuntu:

• Before patch: NS_ERROR_WEBSOCKET_CONNECTION_REFUSED on wss://<host>/websockets, dashboard fails to load.
• After patch: WebSocket upgrade returns HTTP/1.1 101 Switching Protocols, dashboard connects, MATE/XFCE desktop renders, input round-trip works.

Also verified directly:

$ curl -i -H 'Upgrade: websocket' -H 'Connection: Upgrade' \
       -H 'Sec-WebSocket-Key: <16 random bytes b64>' \
       -H 'Sec-WebSocket-Version: 13' \
       https://<host>/websockets
HTTP/1.1 101 Switching Protocols

Source / References:

• Frontend URL construction: addons/selkies-web-core/selkies-ws-core.js upstream of selkies-project/selkies, the line websocketEndpointURL.pathname += 'websockets'.
• Server-side path matching: src/selkies/signaling_server.py upstream, the process_request method that returns None (i.e. accepts WS upgrade) only for /ws, /ws/, /signaling and /signaling/.

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[Copilot]

Pull request overview

This PR updates the bundled nginx default config to support the selkies-dashboard frontend’s WebSocket URL convention by adding a plural /websockets location that proxies to Selkies’ /ws endpoint.

Changes:

• Add location SUBFOLDERwebsockets to proxy WebSocket upgrades to http://127.0.0.1:CWS/ws.
• Apply the same change to both nginx server blocks (ports 3000 and 3001).

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[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/selkies-base/debiantrixie-aa141909-pkg-aa141909-dev-8fa3b6b1a0bb92a3326c8d22f6da590f33a25a1a-pr-151/index.html
https://ci-tests.linuxserver.io/lspipepr/selkies-base/debiantrixie-aa141909-pkg-aa141909-dev-8fa3b6b1a0bb92a3326c8d22f6da590f33a25a1a-pr-151/shellcheck-result.xml

Tag	Passed
amd64-debiantrixie-aa141909-pkg-aa141909-dev-8fa3b6b1a0bb92a3326c8d22f6da590f33a25a1a-pr-151	✅
arm64v8-debiantrixie-aa141909-pkg-aa141909-dev-8fa3b6b1a0bb92a3326c8d22f6da590f33a25a1a-pr-151	✅