dockerTools: unpin go version

The linked PR has been merged, and in fact dockerTools has upgraded to the
latest `go1.11` compiler:

moby/moby#35739
moby/moby#37358

pkgs/top-level/all-packages.nix

@@ -129,8 +129,7 @@ with pkgs;
 
   digitalbitbox = libsForQt5.callPackage ../applications/misc/digitalbitbox { };
 
-  # go 1.9 pin until https://github.com/moby/moby/pull/35739
-  dockerTools = callPackage ../build-support/docker { go = go_1_9; };
+  dockerTools = callPackage ../build-support/docker { };
 
   docker_compose = pythonPackages.docker_compose;