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