hook command is required

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

Author: ndeloof

schema/compose-spec.json

@@ -837,7 +837,8 @@
         "environment": {"$ref": "#/definitions/list_or_dict"}
       },
       "additionalProperties": false,
-      "patternProperties": {"^x-": {}}
+      "patternProperties": {"^x-": {}},
+      "required": ["command"]
     },
 
     "env_file": {