Fix portal docs root path when multiple repositories are checked out

[lbussell]

Fixes #6857

#6857 happens because the --root variable isn't set correctly when multiple repos are checked out, which always happens during publishing.

Related:

• Add variable for repo root in ImageBuilder container image docker-tools#1876 tried to solve this from a different angle, but that PR caused additional issues when flowing into this repo via [nightly] Update common Docker engineering infrastructure with latest #6869. I think that this solution is probably simpler.

[lbussell]

/backport to main

[github-actions]

Started backporting to main (link to workflow run)