The binary name used for executing "containerd publish" was hard-coded
in the shim code, and hence it did not work with customized daemon
binary name. (e.g. `docker-containerd`)
This commit allows specifying custom daemon binary via `containerd-shim
-containerd-binary ...`.
The daemon invokes this command with `os.Executable()` path.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
