GitLab CI: Start docker image from openjdk not maven

[Mihai Nita]

See openjdk, and maven

Seem to have more control and options: Maven: 3.6.0-jdk-8-<platform>, 3.6-jdk-8-<platform>, 3-jdk-8-<platform>, platform : alpine / slim Openjdk: has Java sub-versions (8u171, 8u181, ...) and more platforms (stretch / alpine, full / slim)

And I think it is more important to control the Java version than the maven version.

When using openjdk we can either install the default maven / ant (apt-get), or download - unpack whatever version we want See for instance https://bitbucket.org/nmihai_2000/fake_okapi/src/dev/codeship_tools/install_tools.sh