Maven is well installed on my gitlab-runner server. When executing mvn clean directly on my repo it works, when running my pip
mvn clean
Just add the maven docker image, add below line as first line:
image: maven:latest or image: maven:3-jdk-10 or image: maven:3-jdk-9
refer: https://docs.gitlab.com/ee/ci/examples/artifactory_and_gitlab/