Add support for mac address to docker run configurations

Problem

The docker run option --mac-address was introduced recently (https://docs.docker.com/engine/reference/commandline/run/#options) which gives the ability to assign a mac address to a container. However, with the current docker configurations for GitLab, we do not have the ability to add that parameter (https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersdocker-section)

Proposal

Add the option for mac address in the docker configurations.