Fix bash completion for `docker logs --since`

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -1406,7 +1406,7 @@ _docker_logs() {
 			COMPREPLY=( $( compgen -W "--details --follow -f --help --since --tail --timestamps -t" -- "$cur" ) )
 			;;
 		*)
-			local counter=$(__docker_pos_first_nonflag '--tail')
+			local counter=$(__docker_pos_first_nonflag '--since|--tail')
 			if [ $cword -eq $counter ]; then
 				__docker_complete_containers_all
 			fi