Docker build npm creds

The Docker build needs to access the "@clearlydefined/spdx" package, which is on `npm.pkg.github.com` and requires a login.

See:
https://docs.npmjs.com/docker-and-private-modules#update-the-dockerfile

The same method as #1312 will not work here, as we don't want to copy the `.npmrc` with the token into the container image.