I think "That's true though!" is the best translation of this sentence I've seen. "Doch" always has to be considered in the context of the remark. For example, at the outbreak of WW1, the Austrian Emperor uttered the words "Also doch!" Anyone care to put that into English?
Not necessarily. It can also be to contradict a previous assumption.
Just take the case of a quiz, where your friend has to decide wheter the statue of liberty has the torch in the left or right hand. He decides right, but you are completely convinced he is wrong, and it's the left hand. Now, when reading the solution, you might shout "Oh, das [=rechts] stimmt doch", saying "oh, you were right after all".
(That just was an actual assumption of yesterdays pubquiz. And just to point out, we had the correct solution.)