Replace buildx/login GitHub Actions with plain docker commands

setup-buildx-action downloads from GitHub Releases which is slow
and unreliable from the Forgejo runner. Plain docker build/push
works fine since Docker is already on the runner host.

.forgejo/workflows/build.yaml

@@ -30,23 +30,13 @@ jobs:
           TAG="main-${TIMESTAMP}-${SHORT_SHA}"
           echo "tag=${TAG}" >> $GITHUB_OUTPUT
 
-      - name: Set up Docker Buildx
-        uses: https://github.com/docker/setup-buildx-action@v3
-
       - name: Log in to container registry
-        uses: https://github.com/docker/login-action@v3
-        with:
-          registry: ${{ secrets.REGISTRY_URL }}
-          username: ${{ secrets.REGISTRY_USER }}
-          password: ${{ secrets.REGISTRY_PASSWORD }}
+        run: echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login ${{ secrets.REGISTRY_URL }} -u ${{ secrets.REGISTRY_USER }} --password-stdin
 
       - name: Build and push Docker image
-        uses: https://github.com/docker/build-push-action@v6
-        with:
-          context: .
-          push: true
-          tags: |
-            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.tag.outputs.tag }}
-            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
-          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
-          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache,mode=max
+        run: |
+          IMAGE="${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}"
+          TAG="${{ steps.tag.outputs.tag }}"
+          docker build -t "${IMAGE}:${TAG}" -t "${IMAGE}:latest" .
+          docker push "${IMAGE}:${TAG}"
+          docker push "${IMAGE}:latest"