docker-containerd: add missing docker-containerd symlink

docker-containerd: add missing docker-containerd symlink

Prior to commit 2dccb4f2fefd8a0dbda65dd0f7537f811e920b13
("package/docker-containerd: convert to golang infrastructure"), the
containerd binary was installed as docker-containerd. Following the
conversion to the golang package infrastructure, the binary is now
installed as "containerd", which breaks some use cases.

Let's add a symlink link docker-containerd -> containerd to fix such
use cases.

Fixes: https://bugs.busybox.net/show_bug.cgi?id=11076
Signed-off-by: Sam Lancia <sam@gpsm.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>