Optionally ship required modules for JMX in our Docker image

[robotdan]

Re: #59

We added the option for anyone to add additional JDK modules and build their own image based upon our Dockerfile. This was added to make it easier to add JMX support to the base image. This can be done by setting JDK_MODULES.

If we think that many will want to use JMX, perhaps it makes sense to add these two modules to our base image if it doesn't add much bloat.

At a minimum, we would need to add jdk.management.agent and if we want to support the HTTP connector, we'd also need to add jdk.httpserver.