feat: add CI workflow to build custom BuildKit image via plain docker build

.gitea/workflows/buildkit.yaml

@@ -0,0 +1,29 @@
+name: Build and push buildkit image
+run-name: ${{ gitea.actor }} is building and pushing buildkit image
+on:
+  push:
+    paths:
+      - 'buildkit/**'
+      - '.gitea/workflows/buildkit.yaml'
+
+env:
+  RESULT_IMAGE_NAME: tools/buildkit-with-ca
+
+jobs:
+  build-and-push-image:
+    runs-on: docker
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Log in to registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ vars.PRIVATE_REGISTRY_DOMAIN }}
+          username: ${{ vars.PRIVATE_REGISTRY_USER }}
+          password: ${{ secrets.PRIVATE_REGISTRY_PASSWORD }}
+
+      - name: Build and push image
+        run: |
+          docker build -t ${{ vars.PRIVATE_REGISTRY_DOMAIN }}/${{ env.RESULT_IMAGE_NAME }}:latest buildkit/
+          docker push ${{ vars.PRIVATE_REGISTRY_DOMAIN }}/${{ env.RESULT_IMAGE_NAME }}:latest