Creating a container from a container

[Konrad Hinsen <konrad.hinsen@fastmail.net>]

Hi everyone,

I am trying to run "guix shell -C" inside another container created with
"guix shell -C". I figured out that I need to take care of three
aspects:

 - Include the package "guix" in the outer container.

 - --expose=/gnu/store such that guix-in-the-container can access the
   store

 - --expose=/var/guix/daemon-socket/socket such that
    guix-in-the-container can connect to the daemon

With those precautions, my setup works some of the time but in other
conditions (which I haven't yet identified) I get the error message

  guix shell: error: mount: mount "none" on "/tmp/guix-directory.K4MoFR/sys": Operation not permitted

when I try to run "guix shell" inside the outer container. Browsing the
Guix source code, I found the probable location where this happens:
in (gnu build linux-container), the procedure run-container calls

   (mount-file-systems root mounts
      #:mount-/proc? (memq 'pid namespaces)
      #:mount-/sys?  (memq 'net namespaces))

which in turn does

   ;; A sysfs mount requires the user to have the CAP_SYS_ADMIN capability in
   ;; the current network namespace.
   (when mount-/sys?
     (mount* "none" (scope "/sys") "sysfs"
             (logior MS_NOEXEC MS_NOSUID MS_NODEV MS_RDONLY)))

and that looks suspect in view of my error message. But I have no idea
why this happens, nor what that "network namespace" is or why it is
needed. In case it matters, I use -N for the outer container (in which I
use wget to fetch files), but not for the inner containers (the ones
whose creation causes the error).

Does anybody have an idea of the cause of the error, and ideally a
solution or workaround?

Finally, some background as to why I am trying to do this: my outer
container runs a workflow (using good old "make"), which in turn
contains rules that create containers as well. I cannot run the workflow
plus all of its rules in a single container because different rules require
conflicting packages.

Thanks in advance,
  Konrad