Investigate mvnw and different gitlab-ci.yml configurations

Our current build process can be slow due to running apt-get update and installing utilities (rsync). I'd like to look into whether or not we can run a slimmer docker image (possibly with rsync pre-installed) and use mvnw to pull maven on each run.

So, things to look at:

• openjdk docker image instead of maven
  • would still need to fetch rsync
• mvnw usage in gitlab-ci.yml
• custom docker image which we can build and provide for ourselves for testing (still using mvnw)