Volunteer Plant

Volunteer plants start out as seeds that get carried by the wind or some other method and end up where you don't expect them. This was a pot that had soil and no plants, but then I noticed a little seedling there in Spring and now it's become a lot bigger!

Anybody know what plant this is? I'm guessing that it's a variety of the wild mustard plant, but I'm not sure.

Whatever it is, it's doing well. :)