Provide a Maven install

This would make it simpler for developers to use this image to run or reproduce CI builds locally as otherwise they need to install Java themselves.

Even though Jenkins has currently configured a "Maven" global tool (used in our shared pipeline) to install automatically, I checked and all agents have /home/hudsonagent/maven (currently links to jenkins_root/tools/hudson.tasks.Maven_MavenInstallation/Maven). Thus changing the Jenkinks to not install automatically should work fine.