Pull-through docker image registry wired to `coder.ddev.com`

on staging-coder.ddev.com, in a workspace, the docker registry inappropriately points to using `coder.ddev.com` as the pull-through registry.

```
root@j1-rfay:/etc/docker# cat daemon.json
{
  "registry-mirrors": ["http://coder.ddev.com:5000"],
  "insecure-registries": ["coder.ddev.com:5000"]
}
```

This is built into the shared Dockerfile, and ... it should not be done that way!