Put Drydock build steps into their own group in Harbormaster
Summary: Ref T9252. Move these into a new "Drydock" group.
Test Plan: Clicked "Add Build Step", saw Drydock steps in a Drydock group.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9252
Differential Revision: https://secure.phabricator.com/D14237