linux-container: Add #:child-is-pid1? to ‘eval/container’.

* gnu/system/linux-container.scm (eval/container): Add `#:child-is-pid1?`.

Change-Id: I522783f22a38093d862f5cfad1c68baaacd4ecc5
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/system/linux-container.scm

@@ -356,13 +356,18 @@ Run the container with the given options."))
                          (mappings '())
                          (mounts '())
                          (namespaces %namespaces)
-                         (guest-uid 0) (guest-gid 0))
+                         (guest-uid 0) (guest-gid 0)
+                         (child-is-pid1? #t))
   "Evaluate EXP, a gexp, in a new process executing in separate namespaces as
 listed in NAMESPACES.  Add MOUNTS, a list of <file-system>, and MAPPINGS, a
 list of <file-system-mapping>, to the set of directories visible in the
 process's mount namespace.  Inside the namespaces, run code as GUEST-UID and
 GUEST-GID.  Return the process' exit status as a monadic value.
 
+When CHILD-IS-PID1? is true, and if NAMESPACES contains 'pid', then the child
+process runs directly as PID 1. See the documentation for
+`(@@ (gnu build linux-container) call-with-container)` for further details.
+
 This is useful to implement processes that, unlike derivations, are not
 entirely pure and need to access the outside world or to perform side
 effects."
@@ -406,4 +411,5 @@ effects."
                   #:populate-file-system populate-file-system
                   #:namespaces namespaces
                   #:guest-uid guest-uid
-                  #:guest-gid guest-gid))))))
+                  #:guest-gid guest-gid
+                  #:child-is-pid1? child-is-pid1?))))))