build: ci for docker

.github/workflows/docker.yml

@@ -0,0 +1,28 @@
+name: Docker build & publish
+
+on:
+    push:
+        branches: [ "master" ]
+    pull_request:
+        branches: [ "master" ]
+
+jobs:
+    docker:
+        runs-on: ubuntu-latest
+        steps:
+            - name: Set up Docker Buildx
+              uses: docker/setup-buildx-action@v2
+            - name: Login to Docker Hub
+              uses: docker/login-action@v2
+              with:
+                  username: ${{ secrets.DOCKERHUB_USERNAME }}
+                  password: ${{ secrets.DOCKERHUB_TOKEN }}
+            - name: Build
+              if: ${{ github.event_name == 'pull_request' }}
+              uses: docker/build-push-action@v3
+            - name: Build & publish
+              if: ${{ github.event_name == 'push' }}
+              uses: docker/build-push-action@v3
+              with:
+                  push: true
+                  tags: peerjs/peerjs-server-test:nightly