fix(ci): the image build stops pruning the runner's shared Docker daemon

The build job ran `docker system prune -af` on the runner's shared Docker
daemon. That deletes every image no container is using at that moment,
including images other repos' jobs had just pulled and were about to start.
Jobs with a service container wait ~14s for it to become healthy between
pull and create, and fail with "No such image" when a prune lands inside
that wait (CI-runner #16). The runner host prunes nightly instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -315,12 +315,10 @@ jobs:
           echo "build_key=$BUILD_KEY" >> $GITHUB_OUTPUT
           echo "channel=$CHANNEL" >> $GITHUB_OUTPUT
 
-      - name: Free disk space
-        # Prunes unused containers and images, and caps the BuildKit cache at
-        # 5 GB so the package caches survive but old layers don't pile up.
-        run: |
-          docker system prune -af || true
-          docker builder prune --keep-storage 5g -f || true
+      # No disk-space prune step: the runner's Docker daemon is shared, and
+      # `docker system prune -af` here deleted images other repos' jobs had
+      # just pulled and were about to start (CI-runner #16). The runner host
+      # prunes nightly instead.
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4