Adding publish-docker workflow (#1654)

.github/workflows/publish-docker.yml

@@ -0,0 +1,20 @@
+name: Publish Docker
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  publish-docker:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: docker/build-push-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: excalidraw/excalidraw
+          tag_with_ref: true
+          tag_with_sha: true