Run git init to be able to fetch the default branches

Fixes #1037 (closed).

Verify locally

Build the image before and after you check out this branch:

docker build --build-arg NANOC_ENV=production --build-arg=CI_COMMIT_REF_NAME=main -t docs:latest -f Dockerfile.main .

The first time it will fail with the errors shown above, the second time it will succeed.