Dockery is decent at G, not special. From what I've seen, he's a pretty good pass protector and a decent run blocker when he's got his guy right in front of him. Not real good in space, decent, but not actually dominating at the point of attack. Very reliable and hasn't hardly missed a game at Washington.
He's also benefitted a lot from having Gibbs and Bugel running the offense, and a lot of stability on that OL. Other than Casey Rabach missing a few snaps near the end of last year, the Commanders have started the same 5 offensive linemen, in the same offensive scheme for 32 straight games, I think.