Adds the `nested_containers` bottle flag. On the macOS backend it starts
a rootless podman service inside the bottle and exposes its
Docker-compatible API socket, so the agent still runs `docker` and
`docker compose`. No host daemon socket is mounted and the guest gains
no capabilities; backends that cannot run a guest-local engine reject
the flag in the shared prepare template rather than ignoring it.
Podman rather than rootless Docker because Apple Container's capability
bounding set omits CAP_SYS_ADMIN, which the kernel requires to write a
multi-range uid_map via newuidmap. With no subordinate UID range podman
falls back to a single-UID self-mapping an unprivileged process may
write itself, so the image build strips /etc/subuid and /etc/subgid
entries rather than adding them.
That mapping is also why nested containers are not an isolation layer:
root inside one is the agent user outside it. They are a build/test
convenience; the bottle remains the boundary.
Ports the spike branch onto main, renaming docker_access — it implied
Docker and granted access to nothing on the host — and drops podman
from the derived layer now that every built-in image ships it (#451).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
didericis
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