Error pages do not account for SubsiteDomain

[NightJar]

When I create an error page on a subsite and perform a dev/build, a static file is cached to the assets folder.
This file seems correct, excepting the base tag contains the main site base url, not swapping the domain out for that which the Subsite is set to listen for.

Recreate:

1. Have a project on a domain e.g. localhost
2. create a subsite on this project.
3. set a domain for that subsite e.g. subsite.localhost
4. create an errorpage on that subsite
5. publish that error page
6. perform dev/build
7. see the contents of e.g. public/assets/error-404-subsite.localhost.html contains the base url of http://subsite/ instead of http://subsite.localhost