ci: add Docker.io login step to workflow configuration

.gitea/workflows/bun.yml

@@ -22,6 +22,13 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
+      - name: Login to Docker.io Registry
+        uses: https://git.kjan.de/actions/login-action@9780b0c442fbb1117ed29e0efdff1e18412f7567 # v3
+        with:
+          registry: git.simonis.lol
+          username: ${{ vars.DOCKER_USER }}
+          password: ${{ vars.DOCKER_PASS }}
+
       - name: Build and push Docker image
         uses: docker/build-push-action@v5
         with: