Update CI to work better for external PRs

Author: jackkoenig

.github/workflows/docker.yml

@@ -20,6 +20,7 @@ jobs:
         uses: docker/setup-buildx-action@v1
       - name: Login to DockerHub
         uses: docker/login-action@v1
+        if: github.event_name == 'push'
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_ACCESS_TOKEN }}