Move zsh completion logic to new subcommand: attach

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -507,15 +507,23 @@ __docker_container_commands() {
 }
 
 __docker_container_subcommand() {
-    local -a _command_args opts_help
+    local -a _command_args opts_help opts_attach_exec_run_start
     local expl help="--help"
     integer ret=1
 
+    opts_attach_exec_run_start=(
+        "($help)--detach-keys=[Escape key sequence used to detach a container]:sequence:__docker_complete_detach_keys"
+    )
     opts_help=("(: -)--help[Print usage]")
 
     case "$words[1]" in
         (attach)
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                $opts_attach_exec_run_start \
+                "($help)--no-stdin[Do not attach stdin]" \
+                "($help)--sig-proxy[Proxy all received signals to the process (non-TTY mode only)]" \
+                "($help -):containers:__docker_complete_running_containers" && ret=0
             ;;
         (commit)
             __docker_subcommand && ret=0
@@ -1644,12 +1652,7 @@ __docker_subcommand() {
 
     case "$words[1]" in
         (attach)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                $opts_attach_exec_run_start \
-                "($help)--no-stdin[Do not attach stdin]" \
-                "($help)--sig-proxy[Proxy all received signals to the process (non-TTY mode only)]" \
-                "($help -):containers:__docker_complete_running_containers" && ret=0
+            __docker_container_subcommand && ret=0
             ;;
         (build)
             _arguments $(__docker_arguments) \