ci: add GitHub Actions workflow to build raptoreumd Docker image

Builds multi-arch (amd64 + arm64) image from raptoreum/Dockerfile
and pushes to ghcr.io/cadenadb/raptoreumd:latest on every push to master.

Author: Cadenadb

.github/workflows/docker-build-raptoreumd.yml

@@ -0,0 +1,58 @@
+name: Build and Push Raptoreumd Docker Image
+
+on:
+  push:
+    branches: [ master, main ]
+    paths:
+      - 'raptoreum/Dockerfile'
+      - '.github/workflows/docker-build-raptoreumd.yml'
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: cadenadb/raptoreumd
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU (for multi-arch builds)
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata for Docker
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,prefix=sha-
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: ./raptoreum
+          file: ./raptoreum/Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max