fix: stop CI from filling runner disk

Three sources of unbounded growth removed:
- Drop cache-from/cache-to registry: on a persistent self-hosted runner the
  local BuildKit layer cache already provides between-run reuse; the registry
  cache was redundant and pushed ~2 GB of torch layers on every build
- Switch docker system prune -f → -af so old :SHA-tagged images are removed,
  not just dangling ones (-f alone never touched named tags)
- Add docker builder prune --keep-storage 5g to bound the local BuildKit
  cache; pip mount cache (torch etc.) is recently-used so survives, stale
  intermediate layers are evicted first

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -122,7 +122,12 @@ jobs:
           echo "build_version=$BUILD_VERSION" >> $GITHUB_OUTPUT
 
       - name: Free disk space
-        run: docker system prune -f || true
+        run: |
+          # Remove all unused images (including old :SHA tags) and containers.
+          docker system prune -af || true
+          # Keep the local BuildKit cache bounded so pip mount cache survives
+          # but stale intermediate layers don't accumulate indefinitely.
+          docker builder prune --keep-storage 5g -f || true
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
@@ -142,5 +147,3 @@ jobs:
           provenance: false
           tags: ${{ steps.tags.outputs.value }}
           build-args: BUILD_VERSION=${{ steps.tags.outputs.build_version }}
-          cache-from: type=registry,ref=${{ env.IMAGE }}:cache
-          cache-to: type=registry,ref=${{ env.IMAGE }}:cache,mode=max