Starting from commit 7bf1595c9242 ("isolation: Don't create our userns
as nobody"), we unconditionally set uidmap and gidmap in the detached
user namespace.
If passt is started from a detached PID namespace, but /proc hasn't
been remounted to reflect this, we'll fail to write those entries.
That's actually fine as uidmap and gidmap are something that, strictly
speaking, we only need to write in pasta mode when a command is
detached (it's now done in all cases for simplicity).
Warn, because it's not the expected behaviour (/proc should probably
be remounted first), but don't fail on that.
Link: https://github.com/containers/crun/issues/2283
Suggested-by: David Gibson