Remove volume preventing mount of Docker socket

Merged username-removed-190570 requested to merge lorenz/gitlab-ci-multi-runner:master into master

In the Ubuntu Dockerfile there is the volume /var/run/docker.sock defined. This creates a folder at /var/run/docker.sock and you can't bind-mount the socket anymore because there is now already a folder there.

This MR removes that volume definition and allows you to mount your host Docker socket again.