dev: ci: build Docker images

Uses the existing in-tree Dockerfile to build Docker images when
a release is made and everytime there is a merge to master.
Publishes the images to the GitHub Container Registry as well.
Heavily based on https://docs.docker.com/build/ci/github-actions/examples/

.github/workflows/docker.yml

@@ -0,0 +1,53 @@
+name: Docker images
+
+on:
+  push:
+    branches:
+      - docker
+      - master
+    tags:
+      - '*.*.*'
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=edge
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Login to GitHub Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and Push
+        uses: docker/build-push-action@v4
+        with:
+          contest: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+