Linux containers on FreeBSD

This allows running Linux containers on FreeBSD and modifies the
mounts so that they represent the linux emulated filesystems, as per:
https://wiki.freebsd.org/LinuxJails

Co-authored-by: Gijs Peskens <gijs@peskens.net>, Samuel Karp <samuelkarp@users.noreply.github.com>
Signed-off-by: Artem Khramov <akhramov@pm.me>

oci/spec_opts.go

@@ -377,6 +377,7 @@ func WithImageConfigArgs(image Image, args []string) SpecOpts {
 			return fmt.Errorf("unknown image config media type %s", ic.MediaType)
 		}
 
+		appendOSMounts(s, ociimage.OS)
 		setProcess(s)
 		if s.Linux != nil {
 			defaults := config.Env