Queue Builds so they don't run concurrent

I noticed that pushing to a repo triggers a new build, even when the build of the last build is still running. This should not happen cause it fails because of an existing lockfile:

gitlab-ci-multi-runner 1.3.2 (0323456)
Using Shell executor...
Running on greyhound...
error: could not lock config file /home/gitlab-runner/.gitconfig: File exists

ERROR: Build failed: exit status 1

I set concurrent to 6 but from what i learned this only specifies the number of concurent jobs that can be run in parallel inside a stage.