Pushing to the repo works as expected and I have enabled the "On server clean-up, delete pushed Docker images from registry" on the Docker Support build feature. During the cleanup though I get a 403 error. We are running v2020.2.1 server and agents.
Here is the relevant part of the log:
[2021-01-12 15:12:01,917] INFO - jetbrains.buildServer.CLEANUP - Removing docker images for clean-up builds from remote repositories
[2021-01-12 15:12:02,089] WARN - jetbrains.buildServer.CLEANUP - Unable to remove image 'harbor.app.simplesite.host/devstaging/devstaging:devmachine-1141' from remote registry: Unable to remove image from repo by url: 'https://harbor.app.simplesite.host/v2/devstaging/devstaging/manifests/sha256:70fb59863890194de693ed6d86d0d8c3ef219146880a64612d6a1e09e08f5269': 403(Forbidden)