Extend documentation about accessing docker services

Merged username-removed-36680 requested to merge unused/gitlab-ci-multi-runner:master into master

What does this MR do?

Adds a short note on how the hostname generation convention out from a docker service looks like.

A docker service defined in the .gitlab-ci.yml will be used to define the services hostname (serviceDescription in executors/docker/executor_docker.go). This is perfectly described for using public service, unfortunately not for private service container, especially when they contain a port (from current doc: Everything after : is stripped).

Moving docs to a new location?

Nope sir!

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