Add completion for --userns

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit 3292afe6e6)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/completion.go

@@ -71,6 +71,7 @@ func addCompletions(cmd *cobra.Command, dockerCLI completion.APIClientProvider)
 	_ = cmd.RegisterFlagCompletionFunc("stop-signal", completeSignals)
 	_ = cmd.RegisterFlagCompletionFunc("storage-opt", completeStorageOpt)
 	_ = cmd.RegisterFlagCompletionFunc("ulimit", completeUlimit)
+	_ = cmd.RegisterFlagCompletionFunc("userns", completion.FromList("host"))
 	_ = cmd.RegisterFlagCompletionFunc("volumes-from", completion.ContainerNames(dockerCLI, true))
 }