Fix searxng container permission errors during setup

A fresh `docker compose up -d` shows the searxng container failing its
healthcheck with permission errors at setup (reported in #721 — the
service comes up under names like `odysseus_searxng_1` and never goes
ready, which then blocks the main odysseus container because of the
`depends_on: searxng: condition: service_healthy` gate).

Root cause: the official `searxng/searxng:latest` image runs as the
non-root `searxng` user but its entrypoint still needs to

1. chown /etc/searxng on first boot so the persisted named volume is
   owned by the searxng user inside the container,
2. su-exec to drop / re-assert privileges before launching uwsgi, and
3. let our wrapper entrypoint (which seeds settings.yml into the named
   volume on first boot) write the file through the volume mount.

Without explicit `cap_add`, the container has neither CHOWN nor
DAC_OVERRIDE nor SETUID/SETGID, so the entrypoint aborts at the first
chown / su-exec / redirection with EACCES. The upstream searxng-docker
compose file solves this with the standard "drop everything, grant only
what's needed" capability pattern.

Fix: mirror the upstream cap_drop ALL / cap_add CHOWN, SETGID, SETUID,
DAC_OVERRIDE on the searxng service. This grants only the four caps the
entrypoint actually needs, matches what searxng-docker ships with, and
leaves ports, volumes, env, healthcheck, and the wrapper entrypoint
unchanged.

Closes #721.

docker-compose.yml

@@ -76,6 +76,20 @@ services:
     environment:
       - SEARXNG_BASE_URL=http://localhost:8080/
       - SEARXNG_SECRET=${SEARXNG_SECRET:-}
+    # The official searxng image runs as the non-root `searxng` user, but its
+    # entrypoint still needs to chown /etc/searxng on first boot, drop privs via
+    # su-exec, and (with our wrapper above) write settings.yml into the named
+    # volume. Without these capabilities the wrapper aborts at the redirection
+    # with EACCES and the container fails its healthcheck with permission
+    # errors during setup. Mirrors the cap set recommended by the upstream
+    # searxng-docker compose file. See issue #721.
+    cap_drop:
+      - ALL
+    cap_add:
+      - CHOWN
+      - SETGID
+      - SETUID
+      - DAC_OVERRIDE
     healthcheck:
       test: ["CMD-SHELL", "python -c \"import urllib.request; urllib.request.urlopen('http://localhost:8080/', timeout=5).read(1)\""]
       interval: 5s