Groudon is a weird Pokemon in that despite being almost mandatory on modern Ubers teams (and VGC teams as I understand it, although VGC isn't my area of experience so I can't really comment on it) it's not usually a Pokemon you build around. It's a Pokemon you mold to your team rather than the other way around, so the ideal way to choose a Groudon set is to start building the rest of your team first and see which of the dozen or so viable sets he can run fits into your team the best. Sadly Docile isn't really optimal for any of them, but it's not really detrimental for any of them either.
What set in particular you want to run depends heavily on what you want Groudon to do, as well as the rest of your team. If you need a reliable Stealth Rock setter that can check Xerneas, Arceus-Fairy, Arceus-Water, Kyogre, etc., then a specially defensive set is probably your best bet. If you'd rather run a sweeper variant of Groudon, investing into Attack and Speed and running a set with Swords Dance and either Rock Polish or Thunder Wave can tear teams apart once things like Giratina-O and Lugia are weakened or removed. If you'd rather give Groudon some staying power to more reliably check threats like Xerneas, Mega Salamence, and various Arceus forms, runing a RestTalk set is Groudon's only means of recovery. There's a ton of other options he can run as well, utilizing things such as Overheat, Eruption, Dragon Claw, Rock Tomb, etc.