Support Docker 1.13 --cpus option

Docker 1.13 introduced --cpus option, which

In Docker 1.13 we added a --cpus flag, which is the best tech for limiting cpu usage of a container with a sane UX that the majority of users can understand.

This option is somewhat similar to the cpuset_cpus, but cpuset_cpus limits which cores can be used by Docker; cpus limits how many physical cores are available to the container.

It would be nice, if I could use this option in .gitlab-ci.yml, but if it's faster to introduce it in the global /etc/gitlab-runner/config.toml.