Removed `runAsUser` property from securityContext of gitlab-runner deployment

The entrypoint script (on both ubuntu and alpine images) calls update-ca-certificates to install the user provided CA certificates which fails when run as gitlab-runner user.

This issue and !99 (closed) together cause the runner pod to not work out of the box in an environment with internal CAs.

This PR simply removes the runAsUser directive as the simplest solution to solve this problem. But if for some reason running the gitlab-runner as container root is not an acceptable solution then it is also possible to add a --tls-ca-file <path-to-ca> flag here although I haven't thoroughly tested this.