Use registry build cache instead of gha cache

The GitHub-hosted 'gha' cache backend is unreachable from the
self-hosted Gitea runner (dial tcp 172.18.0.2:44181 i/o timeout), which
failed the build step even though the image pushed successfully. Switch
cache-from/cache-to to a registry-backed cache stored as a :buildcache
tag in the same Gitea registry, which works on self-hosted runners.

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

@@ -46,8 +46,11 @@ jobs:
           tags: |
             ${{ env.REGISTRY }}/${{ gitea.repository_owner }}/blog-ahmza-com:latest
             ${{ env.REGISTRY }}/${{ gitea.repository_owner }}/blog-ahmza-com:${{ gitea.event.head_commit.id }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          # Registry-backed build cache. The GitHub-hosted 'gha' cache is not
+          # reachable from self-hosted Gitea runners, so store the cache as a
+          # dedicated tag in the same registry instead.
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ gitea.repository_owner }}/blog-ahmza-com:buildcache
+          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ gitea.repository_owner }}/blog-ahmza-com:buildcache,mode=max
   deploy:
     runs-on: ubuntu-latest
     needs: build