commands/docker-run: --help works [GH-3698]

CHANGELOG.md

@@ -10,6 +10,7 @@ BUG FIXES:
       Vagrantfile.
   - core: Plugins that define config methods that collide with Ruby Kernel/Object
   -   methods are merged properly. [GH-3670]
+  - commands/docker-run: `--help` works. [GH-3698]
   - commands/package: `--base` works without crashing for VirtualBox.
   - commands/reload: If `--provision` is specified, force provisioning. [GH-3657]
   - guests/redhat: Fix networking issues with CentOS. [GH-3649]

plugins/providers/docker/command/run.rb

@@ -29,18 +29,22 @@ module VagrantPlugins
           # Parse out the extra args to send to SSH, which is everything
           # after the "--"
           split_index = @argv.index("--")
-          if !split_index
-            @env.ui.error(I18n.t("docker_provider.run_command_required"))
-            return 1
+          if split_index
+            command = @argv.drop(split_index + 1)
+            @argv   = @argv.take(split_index)
           end
 
-          command = @argv.drop(split_index + 1)
-          @argv   = @argv.take(split_index)
-
           # Parse the options
           argv = parse_options(opts)
           return if !argv
 
+          # Show the error if we don't have "--" _after_ parse_options
+          # so that "-h" and "--help" work properly.
+          if !split_index
+            @env.ui.error(I18n.t("docker_provider.run_command_required"))
+            return 1
+          end
+
           target_opts = { provider: :docker }
           target_opts[:single_target] = options[:pty]