Fix CI: switch Docker cache from registry to inline

Registry cache export was failing with 500 errors from the container
registry. Inline cache embeds metadata in the image itself, avoiding
the separate cache manifest upload.

.forgejo/workflows/build.yaml

@@ -39,7 +39,6 @@ jobs:
           echo "image=${IMAGE}" >> $GITHUB_OUTPUT
           echo "tag=${IMAGE}:main-${TIMESTAMP}-${SHORT_SHA}" >> $GITHUB_OUTPUT
           echo "latest_tag=${IMAGE}:latest" >> $GITHUB_OUTPUT
-          echo "cache_tag=${IMAGE}:buildcache" >> $GITHUB_OUTPUT
 
       - name: Build and push Docker image
         uses: docker/build-push-action@v6
@@ -49,8 +48,8 @@ jobs:
           tags: |
             ${{ steps.meta.outputs.tag }}
             ${{ steps.meta.outputs.latest_tag }}
-          cache-from: type=registry,ref=${{ steps.meta.outputs.cache_tag }}
-          cache-to: type=registry,ref=${{ steps.meta.outputs.cache_tag }},mode=max
+          cache-from: type=registry,ref=${{ steps.meta.outputs.latest_tag }}
+          cache-to: type=inline
 
       - name: Update deployment image tag
         run: |