docs(ci/docker_build): Add example of variable use
docs(ci/docker_build): Add example of variable use

t-zuehlsdorff/gitlabhq,darkrasid/gitlabhq,dplarson/gitlabhq,htve/GitlabForChinese,axilleas/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,LUMC/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,htve/GitlabForChinese,darkrasid/gitlabhq,LUMC/gitlabhq,mmkassem/gitlabhq,LUMC/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,htve/GitlabForChinese,darkrasid/gitlabhq,dreampet/gitlab,t-zuehlsdorff/gitlabhq,htve/GitlabForChinese,t-zuehlsdorff/gitlabhq,darkrasid/gitlabhq,LUMC/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,dreampet/gitlab,axilleas/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,dplarson/gitlabhq