add Github action to publish docker image (#31)

.github/workflows/docker.yml

@@ -0,0 +1,22 @@
+name: Build and publish to Docker Hub
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: joystream/orion:latest