docker: add clientOnly / docker-client

Currently the docker client is only available on non-linux platforms as `docker`,
this makes the client available on linux and other platforms as `docker-client`.

(cherry picked from commit 7233acd515546107e5902a8fef6832f4f319390c)

pkgs/top-level/all-packages.nix

@@ -23020,6 +23020,7 @@ in
 
   docker = docker_20_10;
   docker-edge = docker_20_10;
+  docker-client = docker.override { clientOnly = true; };
 
   docker-proxy = callPackage ../applications/virtualization/docker/proxy.nix { };