(Port from CE) Document how to specify environment variables during manual… (!7260) · Merge requests · GitLab.org / GitLab · GitLab

Evan Read requested to merge ee-specify-variables-manual-pipeline into master Sep 06, 2018

Porting changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21548.

Please merge first.

Please review, @gl-docsteam.