Install rum agent from apm before creating the docker image

[hmdhk]

Following the image release process we introduced with #27. I think the approach with fetching the agent source from github works well for the latest version since it gives us ability to fix the tests quickly. But for the versions that are published to npm I think it's best to use install them from npm instead of using the github branch. I'm not sure if we can keep both approaches or not but that would be the ideal solution.

cc @v1v