fix(docker): migrate retained SearXNG settings (#6055)

* fix(docker): migrate retained SearXNG settings

Retained nonempty SearXNG settings can miss defaults required by newer pinned images while bypassing the entrypoint's narrow regeneration checks.

Add an atomic PyYAML-aware migration to all Compose variants. Preserve existing inheritance choices, custom content, secrets, ownership, and mode while inserting only the missing top-level default-inheritance key.

Validated with 39 focused and adjacent tests, compile checks, and fresh and retained pinned-image HTTP 200 gates. Full repository CI remains for the PR.

* fix(docker): chmod the settings temp file before chowning it

The Compose cap set is `cap_drop: ALL` plus CHOWN/SETGID/SETUID/DAC_OVERRIDE
and carries no FOWNER, and searxng's own entrypoint chowns /etc/searxng to
searxng:searxng, so every retained settings file belongs to that user by the
second boot. Chowning the temporary file first left root unable to chmod it,
so the migration exited 1 and `set -eu` killed the container before
`exec /usr/local/searxng/entrypoint.sh` — SearXNG never started and odysseus
blocked on its healthcheck.

Swap the two calls so the chmod lands while the temporary file is still
root-owned, and cover the ordering with a test that refuses the chmod once
the chown has happened, the way the kernel does.

* fix(docker): let searxng boot when the settings migration fails

The migration runs under `set -eu`, so any settings file it cannot parse or
rewrite took the container down instead of merely going unmigrated. A symlinked
/etc/searxng/settings.yml is enough: the migration refuses a non-regular file
and searxng, which reads through the symlink perfectly well, never got to start.

Guard the call with `|| true` in all three Compose variants. The failure still
prints its reason on stderr, and searxng is left to report anything genuinely
wrong with the file.

---------

Co-authored-by: Léo <leograndcontact@gmail.com>

docker-compose.gpu-amd.yml

@@ -130,12 +130,17 @@ services:
           fi
           sed "s|__SEARXNG_SECRET__|$$secret|g" /tmp/searxng-settings.yml.template > /etc/searxng/settings.yml
         fi
+        # Advisory: a settings file the migration cannot parse or rewrite must
+        # not be what stops searxng from booting. It explains itself on stderr
+        # and we carry on, letting searxng report anything genuinely wrong.
+        /usr/local/searxng/.venv/bin/python /tmp/migrate-searxng-settings.py /etc/searxng/settings.yml || true
         exec /usr/local/searxng/entrypoint.sh
     ports:
       - "127.0.0.1:8080:8080"
     volumes:
       - searxng-data:/etc/searxng
       - ./config/searxng/settings.yml:/tmp/searxng-settings.yml.template:ro,z
+      - ./scripts/migrate_searxng_settings.py:/tmp/migrate-searxng-settings.py:ro,z
     environment:
       - SEARXNG_BASE_URL=http://localhost:8080/
       - SEARXNG_SECRET=${SEARXNG_SECRET:-}