docker-compose: move condition to package

make the default.nix self-sufficient

docker-compose/default.nix

@@ -1,6 +1,11 @@
 { buildCLIImage
-, docker-compose
+, docker-compose ? null
+, python3Packages
 }:
 buildCLIImage {
-  drv = docker-compose;
+  drv =
+    if docker-compose == null
+    then python3Packages.docker_compose
+    else docker-compose # nixos 19.03+
+    ;
 }