ci: persist BuildKit mount caches across runs (#4421)

* ci: persist BuildKit mount caches across runs

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

* ci: key mount caches on native-build inputs, drop npm/uv from cache-map

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

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: ZaynJarvis

.github/workflows/build-docker-image.yml

@@ -95,8 +95,39 @@ jobs:
             type=raw,value=main,enable=${{ github.ref == 'refs/heads/main' }}
 
       - name: Set up Docker Buildx
+        id: buildx
         uses: docker/setup-buildx-action@v4
 
+      # Persist the Dockerfile's cargo/ccache RUN --mount=type=cache dirs across
+      # runs. On ephemeral runners they start empty, so the Rust CLI and C++
+      # extension recompile from scratch every build (~13 min of a ~16 min job).
+      # Key on the native-build inputs, not the commit sha: commits that do not
+      # touch Rust/C++ hit the exact key, which also skips the cache upload, so
+      # this stays quiet inside the repo's shared 10 GB Actions cache budget.
+      # npm/uv mounts are deliberately not persisted: they only save ~1 min and
+      # would churn the cache on every dependency change.
+      - name: Restore BuildKit mount caches
+        id: mount-cache
+        uses: actions/cache@v4
+        with:
+          path: buildkit-mount-cache
+          key: buildkit-mount-${{ matrix.arch }}-${{ hashFiles('Cargo.toml', 'Cargo.lock', 'crates/**', 'src/**', 'third_party/**', 'setup.py') }}
+          restore-keys: |
+            buildkit-mount-${{ matrix.arch }}-
+
+      - name: Inject mount caches into BuildKit
+        uses: reproducible-containers/buildkit-cache-dance@v3.4.0
+        with:
+          builder: ${{ steps.buildx.outputs.name }}
+          cache-map: |
+            {
+              "buildkit-mount-cache/cargo-target": {"target": "/cargo-target", "id": "cargo-target-${{ matrix.platform }}"},
+              "buildkit-mount-cache/cargo-registry": {"target": "/usr/local/cargo/registry", "id": "cargo-registry-${{ matrix.platform }}"},
+              "buildkit-mount-cache/cargo-git": {"target": "/usr/local/cargo/git", "id": "cargo-git-${{ matrix.platform }}"},
+              "buildkit-mount-cache/ccache": {"target": "/root/.ccache", "id": "ccache-${{ matrix.platform }}"}
+            }
+          skip-extraction: ${{ steps.mount-cache.outputs.cache-hit }}
+
       - name: Build and push Docker image to GHCR
         id: push-ghcr
         uses: docker/build-push-action@v7