Add a release actions

Github workflows to create a nightly release automatically and a
stable release semi-automatic (a 2-step process).

.github/workflows/docker-image.yml

@@ -0,0 +1,22 @@
+name: Publish Docker Images
+on:
+  release:
+    types: [ published ]
+jobs:
+  docker-images:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2.3.4
+        with:
+          fetch-depth: 0
+      - name: Set current version
+        run:  echo "DOCSPELL_VERSION=$(cat version.sbt | grep version | cut -d= -f2 | xargs)" >> $GITHUB_ENV
+      - name: Build Docker Images (${{ env.DOCSPELL_VERSION }})
+        run: ./docker/dockerfiles/build.sh ${{ env.DOCSPELL_VERSION }}
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+      - name: Publish Images (${{ env.DOCSPELL_VERSION }})
+        run: ./docker/dockerfiles/push.sh ${{ env.DOCSPELL_VERSION }}