ci: add Docker BuildKit registry cache to speed up Rust builds

Caches compiled dependency layers in the Gitea registry under
:buildcache. Subsequent builds that only touch solitaire_server/src/
skip recompiling the full workspace dependency tree.

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

.gitea/workflows/docker-build.yml

@@ -36,6 +36,11 @@ jobs:
           username: ${{ gitea.actor }}
           password: ${{ secrets.CI_TOKEN }}
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          driver-opts: network=host
+
       - name: Build and push
         uses: docker/build-push-action@v5
         with:
@@ -45,6 +50,8 @@ jobs:
           tags: |
             ${{ env.IMAGE }}:${{ steps.meta.outputs.sha }}
             ${{ env.IMAGE }}:latest
+          cache-from: type=registry,ref=${{ env.IMAGE }}:buildcache
+          cache-to: type=registry,ref=${{ env.IMAGE }}:buildcache,mode=max
 
       - name: Install kustomize
         run: |