package/docker-containerd: security bump to 1.2.12

package/docker-containerd: security bump to 1.2.12

Fixes the following security issues:

- CVE-2019-19921: runc volume mount race condition with shared mounts

- CVE-2019-16884: runc through 1.0.0-rc8, as used in Docker through
  19.03.2-ce and other products, allows AppArmor restriction bypass because
  libcontainer/rootfs_linux.go incorrectly checks mount targets, and thus a
  malicious Docker image can mount over a /proc directory.

For details, see the announcement:
https://github.com/containerd/containerd/releases/tag/v1.2.12

containerd is now a separate CNCF sponsored project, and is no longer
explicitly associated with docker/moby.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>