docker_18_09: fix build

The cross-compilation patch introduced in
https://github.com/NixOS/nixpkgs/pull/92140
fixes an issue introduced in Docker CE 19.03,
and will not apply to (and is not needed for)
previous versions.

pkgs/applications/virtualization/docker/default.nix

@@ -79,7 +79,7 @@ rec {
       sha256 = sha256;
     };
 
-    patches = [
+    patches = lib.optional (versionAtLeast version "19.03") [
       # Replace hard-coded cross-compiler with $CC
       (fetchpatch {
         url = https://github.com/docker/docker-ce/commit/2fdfb4404ab811cb00227a3de111437b829e55cf.patch;