pkg-export-docker: Unique short for --user-id
This patches fixes the docker exporter breakage from commit by
assigning a yet-unused short option, -s to --user-id.
@@ -149,7 +149,7 @@ impl<'a, 'b> Cli<'a, 'b> {
.arg(
Arg::with_name("USER_ID")
.long("user-id")
- .short("i")
+ .short("s")
.value_name("USER_ID")
.validator(valid_user_id)
.help(
