Add pipeline to build and publish docker image

This MR implements a gitlab pipeline that:

1. test the submodules and report the test coverage
2. build the docker image and publish it to the registry (note: images only build on develop and master branch, other branches need to trigger the job manually)

To make gradlew work, I included a gradle wrapper jar in this MR. You can verify the sha256sum of it and compare it with this page from Gradle: https://gradle.org/release-checksums/ (should be 2db75c40782f5e8ba1fc278a5574bab070adccb2d21ca5a6e5ed840888448046)