package/docker-containerd: security bump to version 1.4.3

package/docker-containerd: security bump to version 1.4.3

Fixes the following security issue:

- CVE-2020-15257: Access controls for the shim’s API socket verified that
  the connecting process had an effective UID of 0, but did not otherwise
  restrict access to the abstract Unix domain socket.  This would allow
  malicious containers running in the same network namespace as the shim,
  with an effective UID of 0 but otherwise reduced privileges, to cause new
  processes to be run with elevated privileges.

For more details, see the advisory:
https://github.com/containerd/containerd/security/advisories/GHSA-36xw-fx78-c5r4

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>