Move zsh completion logic to new subcommand: export

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

contrib/completion/zsh/_docker

@@ -670,7 +670,10 @@ __docker_container_subcommand() {
             esac
             ;;
         (export)
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                "($help -o --output)"{-o=,--output=}"[Write to a file, instead of stdout]:output file:_files" \
+                "($help -)*:containers:__docker_complete_containers" && ret=0
             ;;
         (inspect)
             __docker_subcommand && ret=0
@@ -1930,10 +1933,7 @@ __docker_subcommand() {
             __docker_container_subcommand && ret=0
             ;;
         (export)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                "($help -o --output)"{-o=,--output=}"[Write to a file, instead of stdout]:output file:_files" \
-                "($help -)*:containers:__docker_complete_containers" && ret=0
+            __docker_container_subcommand && ret=0
             ;;
         (history)
             _arguments $(__docker_arguments) \