Hi Ross,
That's great! Depending on what you were planning to do (have a look, or start hacking on it?), if (at first) it's only playing with it, then you might want to use the dedicated docker image:
$ docker run -p 8080:8080 jenkinsci/blueocean
About your local error, I think you're just trying to build a maven module of the whole repo, but that plugin actually depends on some others.
So, first get back to the root of the repo, and run
$ mvn clean install
That will build and install all the plugins inside what's the local maven cache (or somewhat improperly /repository/, but really it's better to see it as a cache. Anyway).
Then run hpi:run like you did.
Cheers!