Improve private container registries documentation
Follow-up for https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/commit/3ac8231492c00dbf95560a3301b21fe368b02769#note_36170595:
We should add information that the ~/.docker/config.json
file needs to be created for a user who owns Runner's process (and notice that this is root
for the installation from our official package). And I think it would be also a good idea to include the description of difference between Docker and Shell executors. While using any other type of executor (like Virtualbox), which uses an external machine, it's obvious that we need to prepare configuration there, in case of using both Docker and Shell executors on the same host someone could assume, that the configuration needs to be added only in one place.