Docker build npm creds

[jeffmendoza]

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.

[jeffmendoza]

On second thought, @clearlydefined/spdx should be able to be installed by anyone without a token, so another method should be tried.