Update release.yml

Add git checkout

Author: seanpmorgan

.github/workflows/release.yml

@@ -102,6 +102,7 @@ jobs:
     runs-on: ubuntu-18.04
     if: (github.event_name == 'push' && github.ref == 'refs/heads/master')
     steps:
+      - uses: actions/checkout@v2
       - run: |
           set -e -x
           docker login --username ${{ secrets.DOCKER_USER }} --password ${{ secrets.DOCKER_PW }}