ci: drop duplicate docker.sock mount in build-iso

Forgejo-runner's valid_volumes already injects /var/run/docker.sock
into every job container, so the explicit `container.volumes` mount
in the workflow triggered 'Duplicate mount point' and the job never
started. Removed — DOCKER_HOST env is enough.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.forgejo/workflows/build-iso.yml

@@ -16,14 +16,10 @@ jobs:
   build-iso:
     runs-on: ubuntu-latest
     timeout-minutes: 30
-    # The runner is configured for docker-outside-of-docker: it mounts the
-    # host's /var/run/docker.sock. Jobs get the same socket so the
-    # `docker run` inside build.sh talks to the host daemon directly,
-    # no DinD gymnastics.
-    container:
-      image: catthehacker/ubuntu:act-latest
-      volumes:
-        - /var/run/docker.sock:/var/run/docker.sock
+    # The runner is configured for docker-outside-of-docker: its
+    # `valid_volumes` whitelists /var/run/docker.sock and it mounts the
+    # socket into every job container automatically — no need to repeat
+    # that mount here (it'd be a duplicate-mount error).
     env:
       DOCKER_HOST: unix:///var/run/docker.sock
     steps: