No (easy) way how to build the Docker image

[mvinkler]

This app is a showcase for Knative. One should be able to build the Docker image of the app using kn build, with both supported build strategies, using JVM mode or native build mode.

Right now, it is not possible to do this easily as there is the frontend dependency which is available only after building it locally (make frontend) in the local maven repo. kn build however cannot access the local maven repo and fails to fetch the artefact.