Since these are the user login containers, having the ability to add packages
is useful (e.g. I quite often find I want strace).
Doing this requires that we not share `/var` with the login containers since we
want the apk database therein. Previously it was thought that the containers
might need some parts of `/var` for `ctr` to work (e.g. `/var/lib/containerd`)
but this is not the case now (if it ever was) based on my testing.
Fixes#2206.
Signed-off-by: Ian Campbell <ijc@docker.com>
c48acd5d9d