services: docker: Fix missing containerd-shim binary.

This commit fixes error 'time="2020-10-16T…" level=error msg="Handler for POST
/v1.40/containers/…/start returned error: failed to start shim: exec:
\"containerd-shim\": executable file not found in $PATH: unknown"'.

* gnu/services/docker.scm
(containerd-shepherd-service): Add "containerd-shim" to PATH.