Grant searxng DAC_OVERRIDE so it can write its own config

SearXNG never started on rootless Docker — base install or instance, it
restart-looped from the first boot and the web UI was never reachable.

Cause is a specific capability combination, not a general permissions problem.
The image's entrypoint runs as root and, on first start, chowns the mounted
config dir to searxng:searxng (uid 977) using CHOWN, then immediately copies
settings.yml into the directory it has just given away. The compose dropped ALL
capabilities and added back only CHOWN, SETGID and SETUID — so the chown
succeeded and the write did not, because root cannot write to a 0755 directory
owned by another uid without DAC_OVERRIDE:

  cp: can't create '/etc/searxng/settings.yml': Permission denied
  !!! "/etc/searxng/settings.yml" is not a valid file, exiting...

Isolated the cause by running the image against fresh directories: it fails
with the shipped cap set and succeeds with DAC_OVERRIDE added, everything else
identical. Granting CHOWN without DAC_OVERRIDE is the exact breaking pair; all
other capabilities stay dropped.

Verified end to end: base install and a --local instance both come up and serve
200 on their own random ports, each with its own settings.yml and a distinct
secret_key. This also completes the instance verification that was blocked
earlier — searxng could not be runtime-tested while the app could not boot.
Both test installs were removed afterwards.

Separate pre-existing bug, not fixed here: the install hook applies the theme
with sed s/simple_style: auto/.../ but the generated settings.yml contains no
style keys at all, so the substitution matches nothing and CFG_SEARXNG_THEME
has never had any effect. Fixing it means inserting a ui: block rather than
replacing a line, which is a different mechanism and wants its own change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

containers/searxng/docker-compose.yml

@@ -30,6 +30,18 @@ services:
       - CHOWN
       - SETGID
       - SETUID
+      # DAC_OVERRIDE is required, not optional hardening slack. The image's
+      # entrypoint runs as root and, on first start, chowns the mounted config
+      # dir to searxng:searxng (uid 977) using CHOWN — then immediately copies
+      # settings.yml into the directory it just gave away. Without DAC_OVERRIDE
+      # root cannot write to a 0755 directory owned by another uid, so the copy
+      # failed and the container exited with:
+      #   cp: can't create '/etc/searxng/settings.yml': Permission denied
+      #   !!! "/etc/searxng/settings.yml" is not a valid file, exiting...
+      # It restart-looped forever and SearXNG never came up at all. Granting
+      # CHOWN without DAC_OVERRIDE is the specific combination that breaks;
+      # every other capability stays dropped.
+      - DAC_OVERRIDE
     logging:
       driver: "json-file"
       options: