Support devcontainer.json in Cloud Environments (and local Docker)

Cloud Environments currently only let you configure env vars and a bash setup script. There's no way to reuse a repo's existing .devcontainer/devcontainer.json or point at a prebuilt container image.

I already have a devcontainer that Codespaces and VS Code use for my toolchain. To get the same setup in a Claude environment I have to rewrite all of it as a bash script and keep the two in sync manually.

Would be great if Claude Code could just read devcontainer.json from the repo (or let me specify a base image) so the environment lives in one place instead of being duplicated in a setup script. Ideally this works both for cloud environments and locally via Docker.