Follow-up from "Add ability to skip gitaly, workhorse and shell setup via env variables"

The following discussion from !3335 (merged) should be addressed:

• @acunskis started a discussion: (+5 comments)

  I'm not sure if these should be documented, it's a fairly specific use case

Follow up from mr which added ability to skip some components build via environment variable. Instead, these should be moved to gdk configuration file.