Set default, type, and help method for engine in argparse

Fixes --engine cli option not showing up

repo2docker/__main__.py

@@ -217,7 +217,12 @@ def get_argparser():
         "--cache-from", action="append", default=[], help=Repo2Docker.cache_from.help
     )
 
-    argparser.add_argument("--engine", help="Name of the container engine")
+    argparser.add_argument(
+        "--engine", 
+        type=str, 
+        default="docker", 
+        help=Repo2Docker.engine.help,
+    )
 
     return argparser