Wouldn’t it be nice if services like Codespaces or Coder or Gitlab would allow you to target running on their hosted/integrated platform, or let you launch that same container completely locally? Sometimes I wanna take my “remote” dev environment off-line but still benefit from the integrated UX.

This exists. It's called devcontainers and there is a cli for managing it locally.

https://github.com/devcontainers/ https://containers.dev/

If you can express that operation in Terraform, then Coder would let you do that. First problems I can think of are connectivity from the Coder provisioner to your local machine (Tailscale? Local?), and migrating disk images if you want to actually switch a workspace between environments (local provisioner could do this, but no matter what it’ll be slow and janky).

Maybe I don't understand but why doesn't Gitlabs self hosted setup work?