CI: update actions/checkout action to v3 (#48)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

.github/workflows/docker.yml

@@ -9,7 +9,7 @@ jobs:
     if: ${{ github.event.workflow_run.conclusion == 'success' }}
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Install Go
         uses: actions/setup-go@v3
         with: