Add checkout to github workflow

Author: benrowe

.github/workflows/build.yml

@@ -11,11 +11,12 @@ jobs:
     name: Deploy
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v2
       - name: Build
         uses: docker/build-push-action@v1
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_PASSWORD }}
           repository: benrowe/mysql-to-sqlite3
           tags: latest
-