There is an existing rc init script, /etc/init.d/devfs, that is supposed to mount devfs at sysinit. It appears that it is affected by the very same problem that the Alpine image had (there was a post on this that I lost and cannot find), the init script has the keyword ‘-lxc’, indicating it should never run inside a container. Here’s how I fixed it:
# Copyright (c) 2007-2015 The OpenRC Authors.
# See the Authors file at the top-level directory of this distribution and
# This file is part of OpenRC. It is subject to the license terms in
# the LICENSE file found in the top-level directory of this
# distribution and at https://github.com/OpenRC/openrc/blob/master/LICENSE
# This file may not be copied, modified, propagated, or distributed
# except according to the terms contained in the LICENSE file.
description="Set up the /dev directory"
# defaults: keyword -docker -lxc -prefix -systemd-nspawn -vserver
keyword -docker lxc -prefix -systemd-nspawn -vserver
Do we want /dev/shm to be available by default?