I'd like to use this tool as a breakpoint into a container. e.g.
dagger Dockerfile L4
and it pops open a shell at that point. Then I don't need new syntax. earthly has something like this. it's wonderful.
https://docs.earthly.dev/docs/guides/debugging
there's also https://github.com/ktock/buildg
You can call 'dagger -i' to enable interactive debugging. If a step in your pipeline fails, you get an interactive terminal to inspect the current state.