fix: docker build out-of-disk issue - aggressive cleanup

- .dockerignore: exclude build/cache/docs to reduce build context (59 lines)
- Dockerfile: add aggressive cleanup (cache, registry, git) after cargo build
- Dockerfile: use --locked flag for reproducible builds
- build.yaml: add pre-build disk cleanup (docker + cargo cache)
- build.yaml: clean cargo before Docker build to free space
- build.yaml: show disk usage before/after for debugging

Fixes 'No space left on device' error during Docker build in runner

.gitea/workflows/build.yaml

@@ -18,6 +18,15 @@ jobs:
     name: CI
     runs-on: rust
     steps:
+      - name: Clean disk space (runner GC)
+        run: |
+          df -h /
+          echo "Cleaning docker, cargo cache..."
+          docker system prune -af --volumes || true
+          rm -rf ~/.cargo/registry/cache ~/.cargo/registry/index ~/.cargo/git || true
+          rm -rf /tmp/* || true
+          df -h /
+
       - name: Install Node.js and Docker
         run: |
           apt-get update
@@ -48,6 +57,12 @@ jobs:
           REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
+      - name: Clean cargo before Docker build
+        run: |
+          cargo clean || true
+          rm -rf ~/.cargo/registry/cache ~/.cargo/registry/index ~/.cargo/git || true
+          df -h /
+
       - name: Build and push Docker image (SHA tag only)
         run: |
           docker build --no-cache --progress=plain \
@@ -56,5 +71,8 @@ jobs:
           docker push "${IMAGE}:${{ steps.sha.outputs.short_sha }}"
           echo "Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
 
-      - name: Prune unused images
-        run: docker image prune -a --force 2>&1 | tail -3 || true
+      - name: Prune unused images and cleanup
+        run: |
+          docker image prune -a --force 2>&1 | tail -3 || true
+          cargo clean || true
+          df -h /