Verify the container image actually starts

[jboynes]

Use the verify phase to run integration tests on the container.

[ludoch]

Via travis? See https://github.com/GoogleCloudPlatform/appengine-java-vm-runtime/issues/64

[jboynes]

I'll start by making "mvn verify" perform an integration test using the container. The way I did that for the tomcat image was to do the docker build in the packaging phase and then use the integration-test and verify phases to actually test the imaged. That means you can also check changes before committing.

I used the hack with a userland kernel to get this working but if Travis now has built-in docker support that should be easier.