[meta] Do less work when fixing the Dock

ansible/roles/osx/tasks/dock.yml

@@ -1,7 +1,12 @@
 ---
 - homebrew: name=dockutil state=present
+- command: /usr/local/bin/dockutil --list
+  register: dockutil_result
 - command: /usr/local/bin/dockutil --remove all
-- command: /usr/local/bin/dockutil --add ~/Downloads --view list
+  when: not (dockutil_result.stdout_lines | length == 1 and dockutil_result.stdout | search("^Downloads"))
+- command: /usr/local/bin/dockutil --add ~/Downloads
+                                      --view list
                                       --display stack
                                       --sort datemodified
+  when: not (dockutil_result.stdout_lines | length == 1 and dockutil_result.stdout | search("^Downloads"))
 - homebrew: name=dockutil state=absent

ansible/roles/osx/tasks/main.yml

@@ -2,6 +2,7 @@
 - include: bootstrap.yml
 - include: formulae.yml
 - include: dock.yml
+  tags: dock
 - include: casks.yml
 - include: fonts.yml
 - include: defaults.yml