/bin/bash: line 21: mvn: command not found -//Gitlab-ci-multi-runner Container

Hey there , I want to run a gitlab project with java and maven so we can work in a group on our new student project. Currently there are two container running on my vm, a gitlab (GitLab 8.7.5) container and a gitlab-ci-multi-runner container. After I registered the new container as a runner to gitlab and I created a .gitlab-ci.yml file with the following content:

"maven-build: script: - mvn clean integration-test"

But received the following message:

" gitlab-ci-multi-runner 1.2.0 (3a4fcd46) Using Docker executor with image ruby:2.1 ... Pulling docker image ruby:2.1 ... Running on runner-fd61f1aa-project-3-concurrent-0 via d5d2f3f22852... Fetching changes... HEAD is now at 5fa3d7e Update .gitlab-ci.yml Checking out 5fa3d7eb as master... $ mvn clean integration-test /bin/bash: line 21: mvn: command not found

ERROR: Build failed: exit code 1"

I installed maven in the gitlab-ci-multi-runner but still receive the same message.

After two days of searching I did not found anything to solve this problem, so I tried to run a gitlab runner in my gitlab container. The registration seemed to be successful, it shows up in the runners section too, but when I try to built the same .gitlab-ci.yml file Gitlab won't recognize it as a runner....

Is there any easy way to solve this probem ^^? Please help and thanks for your help.