Proxy configuration docs missing NO_PROXY instructions

Problem to solve

The GitLab Runner docs covering how to use the runner behind a proxy are missing instructions for using the NO_PROXY variable. Setting HTTP_PROXY and HTTPS_PROXY without setting NO_PROXY will leave self managed users without externally accessible urls unable to connect to their own GitLab instances. The proxy docs should be updated to make clear that the NO_PROXY variable is available and explain how to use it.

Further details

A user recently ran into an issue while configuring their runners to be able to connect to GitLab.com in order to pull CI templates. After setting the HTTP_PROXY and HTTPS_PROXY they were unable to connect to their local GitLab instance. They needed to set the NO_PROXY variable in order to restore the connection from their runner to their in house GitLab instance. The docs in question didn't properly cover that scenario and they had to do significant troubleshooting.

Proposal

Update the proxy docs to include an explanation of how to use the NO_PROXY variable. The doc should cover the how and why of setting the variable.

Who can address the issue

Anyone

Other links/references

An article on using the various proxy control variables