ci: ensure that the builder container is healthy

GitHub Actions sometimes fails due to unclear reasons with the following
error:

```
$ ./build-exec bundle config set --local without check
There was an error while trying to write to `/home/me/build/.bundle/config`. It
is likely that you need to grant write permissions for that path.
```

This commit tries to fix this by waiting for the builder container

Author: kateinoigakukun

.github/workflows/build.yml

@@ -126,7 +126,8 @@ jobs:
             ${{ steps.builder-image.outputs.imageid }} /bin/sh
           echo 'docker exec -u me builder "$@"' > ./build-exec
           chmod +x ./build-exec
-      - run: ./build-exec bundle config set --local without check
+          # wait for docker exec to be ready
+          timeout 10 bash -c 'until ./build-exec bundle config set --local without check; do sleep 1; done'
       - run: ./build-exec ./bin/setup
       - run: ./build-exec bundle exec rake compile
       - name: Pre-release configuration