1 - This is a security settings problem. In the security headers snippet (in caddy), remove the X-frame options setting, or set a more permissive value. (I’ve reported it, and the team has an issue open internally. Preview post and visit site do not work (Ghost 6.0 container) )
2 - If you haven’t already, can you bump Ghost to 6.0.3 (hopefully a fresh pull of the container will get you that). There was a missing redirect that was impacting specifically sites with separate admin domains, and it is fixed at 6.0.3.
Check your browser’s network console for errors also - any 401s?
I had very similar behavior (without docker) and fixed it with a redirect not-admin-site.tld/ghost/* over to actual-admin-site.tld/ghost/, but that should be fixed and working in 6.0.3.
Another possibility is that your containers aren’t talking to each other correctly