ci: cache container images to GitHub Container Registry

These image builds have been reliably failing recently. Not sure why.
More upstream rate limiting?

Let's add container registry based caching to image builds using
the GitHub Container Registry.

Author: indygreg

.github/workflows/linux.yml

@@ -52,6 +52,8 @@ jobs:
           - xcb
           - xcb.cross
     runs-on: ubuntu-22.04
+    permissions:
+      packages: write
     steps:
       - uses: actions/checkout@v3
 
@@ -60,9 +62,41 @@ jobs:
         with:
           python-version: '3.11'
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Generate Dockerfiles
+        run: |
+          ./build-linux.py --make-target empty
+
       - name: Build Image
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: build/${{ matrix.image }}.Dockerfile
+          labels: org.opencontainers.image.source=https://github.com/${{ github.repository }}
+          # Cache from/to the current branch of the current repo as the primary cache key.
+          # Cache from the default branch of the current repo so branches can have cache hits.
+          # Cache from the default branch of the canonical repo so forks can have cache hits.
+          # Ignore errors on cache writes so CI of forks works without a valid GHCR config.
+          cache-from: |
+            type=registry,ref=ghcr.io/indygreg/${{ github.repository }}:${{ matrix.image }}-${{ github.ref_name }}
+            type=registry,ref=ghcr.io/indygreg/${{ github.repository }}:${{ matrix.image }}-main
+            type=registry,ref=ghcr.io/indygreg/python-build-standalone:${{ matrix.image }}-main
+          cache-to: |
+            type=registry,ref=ghcr.io/indygreg/${{ github.repository }}:${{ matrix.image }}-${{ github.ref_name }},ignore-errors=true
+          outputs: |
+            type=docker,dest=build/image-${{ matrix.image }}.tar
+
+      - name: Compress Image
         run: |
-          ./build-linux.py --make-target toolchain-image-${{ matrix.image }}
           zstd -v -T0 -6 --rm build/image-*.tar
 
       - name: Upload Docker Image