TLDR: I would not recommend Harbormaster.
Phabricator does not have an easy way (like Docker container) to install and test it locally. It does have a free hosted version (trial expires after 7 days). Creating a test hosted instance was easy, a few clicks (test-eqazkbf4yyol).
Bad news start with Harbormaster. Looks like the only thing it supports is running the build at an external CI system, like Jenkins.
A couple of quotes from the documentation:
The current version of Harbormaster can perform some basic build tasks, but has many limitations and is not a complete build platform.
Currently, the only useful type of build step is "Make HTTP Request", which you can use to make a call to an external build system like Jenkins.
@20after4, you have way more experience with Phabricator than I do. Please correct me if I'm wrong about Harbormaster.