Interesting to learn about other related tools. I built a similar variant called ctenv (https://github.com/osks/ctenv). Focused more general containers and not specific to agents, but I'm using it for that via its configurability.

One thing I wanted was to use any image in the container, which shai also seem to support in the same way (mounting a custom entrypoint script). And same reason for not using devcontainers - make it easy to start a new container.