CI: add Docker BuildX caching

.github/workflows/ci.yml

@@ -46,8 +46,11 @@ jobs:
         uses: docker/build-push-action@v4
         with:
           platforms: ${{ matrix.platform }}
+          pull: true
           push: ${{ (github.ref == 'refs/heads/master') && (github.repository == 'citra-emu/citra-multiplayer-dedicated') }}
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
       - name: Export digest
         run: |
           mkdir -p /tmp/digests