revert: remove registry cache backend, keep native Docker layer cache

--cache-to with docker driver doesn't support registry backend.
Host daemon already persists build layers via shared socket.
No changes needed - cache works automatically.

.gitea/workflows/release.yml

@@ -109,13 +109,9 @@ jobs:
 
       - name: Build test-capable image
         if: steps.commit.outputs.committed == 'true'
-        env:
-          DOCKER_BUILDKIT: 1
         run: |
           docker build \
             --target test-runner \
-            --cache-from type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache-test \
-            --cache-to type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache-test,mode=max \
             --build-arg GIT_COMMIT=${{ github.sha }} \
             -t ${{ env.IMAGE_NAME }}-test:${{ steps.version.outputs.new_version }} \
             .
@@ -131,13 +127,9 @@ jobs:
 
       - name: Build production image
         if: steps.commit.outputs.committed == 'true'
-        env:
-          DOCKER_BUILDKIT: 1
         run: |
           docker build \
             --target runner \
-            --cache-from type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache \
-            --cache-to type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache,mode=max \
             --build-arg GIT_COMMIT=${{ github.sha }} \
             -t ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.version.outputs.new_version }} \
             -t ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest \