We are considering allowing the add-on author to automatically publish an add-on (whether made with Maven or not) as a maven artifact from the Directory in a separate Vaadin add-on repository. The author should also be able to choose not to publish it that way, or to publish the add-on himself to Maven central or elsewhere.
Some add-ons made by Vaadin Ltd will probably be published to Maven central, and JPAContainer would be a good candidate, but this is not scheduled yet.
Two alternatives for now: either install the JPAContainer JAR yourself to your local repository, or build the whole JPAContainer project instead of just the demo project. To do the latter (which actually also installs the JPAContainer add-on in your local Maven repository), see the README file in the JPAContainer parent project.