Add GitHub workflow

.github/workflows/docker-build.yml

+on:
+  push:
+    branches: [ mdbook ]
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    permissions:
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 10
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Login to ghcr registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Prepare version info
+        run: |
+          echo "LATEST_COMMIT_SHA=$(git rev-parse --short HEAD)" >> $GITHUB_ENV
+
+      - name: Build and push Docker images
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: ./Dockerfile
+          tags: |
+            ghcr.io/marcobuster/sweng:${{ env.LATEST_COMMIT_SHA }}
+            ghcr.io/marcobuster/sweng:latest
+          push: true