Add GitHub actions build for docker image

  * Add GitHub Actions build for docker image on docker hub.
  * Creates `rpki/stayrtr`, `rpki/rtrmon`, and `rpki/rtrdump` images
    (not all repositories exist at the moment).

.github/workflows/docker.yml

@@ -0,0 +1,58 @@
+name: CI to Docker Hub
+on:
+  push:
+    branches: [master]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check Out Repo
+        uses: actions/checkout@v2
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_HUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_ACCESS_TOKEN }}
+
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Build and push (stayrtr)
+        id: docker_build_stayrtr
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile.stayrtr.prod
+          push: true
+          tags: rpki/stayrtr:latest
+
+      - name: Image digest (stayrtr)
+        run: echo ${{ steps.docker_build_stayrtr.outputs.digest }}
+
+      - name: Build and push (rtrmon)
+        id: docker_build_rtrmon
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile.rtrmon.prod
+          push: true
+          tags: rpki/rtrmon:latest
+
+      - name: Image digest (rtrmon)
+        run: echo ${{ steps.docker_build_rtrmon.outputs.digest }}
+
+      - name: Build and push rtrdump
+        id: docker_build_rtrdump
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile.rtrdump.prod
+          push: true
+          tags: rpki/rtrdump:latest
+
+      - name: Image digest (rtrdump)
+        run: echo ${{ steps.docker_build_rtrdump.outputs.digest }}