Adding Docker support

[sopitz]

I wanted to try out Communote but didn't feel like installing it on my machine directly. I quickly added some docker support. Is that worth keeping for you? Then I would beautify it a bit.

Feedback very welcome!

[rwi]

@sopitz this is a great idea and it's definitely worth keeping 👍

When beautifying it could you please also have a look at the following things:

• the Communote tgz file shouldn't be added via ADD because binaries tend to bloat git repositories terribly. It is also not so easy to maintain. An alternative could be to use curl or wget from a RUN command (like in https://bitbucket.org/atlassian/docker-atlassian-confluence-server/src/b7b21067a5b549038d60fdb7af56bec91bc45362/Dockerfile?at=master&fileviewer=file-view-default) and download the file directly from GitHub.
• you can put all files in the directory build/docker because the Dockerfile can be used to build the docker image
• a short README, describing what it is and how to use it, would be nice 😉

Since the tomcat Docker image is based on an Open JDK Java image we should probably also have a hint in the README explaining that it is beta/not meant for production use because we officially only support Oracle's JRE and haven't tested with Open JDK.

[sopitz]

ok. I'll refactor it and update the pull request then.

the add will surely be replaced, and I'll also try'n use a non-tomcat-base image, it was just to put it together initially (it had everything installed already :D)