Add `-u|--user` flag to docker exec for running command as a different user

Signed-off-by: Lei Jitang <leijitang@huawei.com>

contrib/completion/bash/docker

@@ -407,7 +407,7 @@ _docker_events() {
 _docker_exec() {
 	case "$cur" in
 		-*)
-			COMPREPLY=( $( compgen -W "--detach -d --help --interactive -i -t --tty" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "--detach -d --help --interactive -i -t --tty -u --user" -- "$cur" ) )
 			;;
 		*)
 			__docker_containers_running

docs/sources/reference/commandline/cli.md

@@ -1121,6 +1121,7 @@ You'll need two shells for this example.
       -d, --detach=false         Detached mode: run command in the background
       -i, --interactive=false    Keep STDIN open even if not attached
       -t, --tty=false            Allocate a pseudo-TTY
+      -u, --user=                Username or UID (format: <name|uid>[:<group|gid>])
 
 The `docker exec` command runs a new command in a running container.