fix(ci): check out the repo before building the Docker image

The docker job in the release workflow had no checkout step, so the
Dockerfile was missing. Add actions/checkout.

Author: wavyx

.github/workflows/release.yml

@@ -68,6 +68,8 @@ jobs:
     needs: release # build from the just-published npm version
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
+
       - name: Resolve version
         id: ver
         run: echo "v=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"