[Add Docker Image Build Workflow]

Details:
- Triggers when a PR with label CI:Build is merged
- Builds a new docker image of the given project
- Uploads it to dockerhub
- Move pre-existing workflow to .bk

.github/workflows/ci-build.yml

@@ -0,0 +1,29 @@
+name: CI Build
+
+on:
+  pull_request:
+    types: [closed]
+
+jobs:
+  build:
+    if: github.event.pull_request.merged == true && contains(github.event.pull_request.labels.*.name, 'CI:Build')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ secrets.DOCKERHUB_USERNAME }}/go-ethereum-limechain:${{ github.sha }}