Move zsh completion logic to new subcommand: cp

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

contrib/completion/zsh/_docker

@@ -536,7 +536,20 @@ __docker_container_subcommand() {
                 "($help -): :__docker_repositories_with_tags" && ret=0
             ;;
         (cp)
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                "($help -L --follow-link)"{-L,--follow-link}"[Always follow symbol link]" \
+                "($help -)1:container:->container" \
+                "($help -)2:hostpath:_files" && ret=0
+            case $state in
+                (container)
+                    if compset -P "*:"; then
+                        _files && ret=0
+                    else
+                        __docker_complete_containers -qS ":" && ret=0
+                    fi
+                    ;;
+            esac
             ;;
         (create)
             __docker_subcommand && ret=0
@@ -1699,20 +1712,7 @@ __docker_subcommand() {
             esac
             ;;
         (cp)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                "($help -L --follow-link)"{-L,--follow-link}"[Always follow symbol link]" \
-                "($help -)1:container:->container" \
-                "($help -)2:hostpath:_files" && ret=0
-            case $state in
-                (container)
-                    if compset -P "*:"; then
-                        _files && ret=0
-                    else
-                        __docker_complete_containers -qS ":" && ret=0
-                    fi
-                    ;;
-            esac
+            __docker_container_subcommand && ret=0
             ;;
         (create)
             _arguments $(__docker_arguments) \