I just took a second look at the repo, and realized there's no package.json. Typically, an open-source JavaScript project would have one. It would enumerate the dependencies and how they work together to generate the final output.
As a JavaScript contributor, I expect to be able to type
yarn
yarn run build
I suspect the interactive web is not the author's primary platform. That's fine - it's his project; he can structure it however he wants. I was just surprised to see an unfamiliar, nontrivial dependency to concatenate a handful of lines into a bookmarklet.
Since you asked "Do you know a better way?" you can port the `npm install` line to a dictionary of dependencies in package.json. You can use wireit in that same file to chain the CLIs together, accomplishing what's currently the ENTRYPOINT line in Dockerfile.
It does the same thing that the current setup does, but it follows the conventions of the ecosystem. This makes it more available to external contributors, because they don't have to learn/install tools like Docker that aren't typically seen in JS.