build: Limit deploy to master branch

.github/workflows/ci.yml

@@ -9,9 +9,6 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-    env:
-      DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
-      DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
     steps:
       - uses: actions/checkout@v2
       - name: install go
@@ -20,6 +17,13 @@ jobs:
           go-version: 1.15
       - name: build and test
         run: make
+  deploy:
+    if: ${{ github.ref == 'refs/heads/master' }}
+    runs-on: ubuntu-latest
+    env:
+      DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
+      DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
+    steps:
       - name: enable experimental docker features
         run: |
           echo '{"experimental":true}' | sudo tee /etc/docker/daemon.json