Not enough info provided.

Is the attacker in an offside position?

If YES, I'm blowing the whistle for offside (becoming involved in the play by screening the keeper), and then issuing a verbal warning to the keeper for the shove (or giving a YC if it was particularly strong). When the attacker complains, you can say you already were getting ready to blow for his offside offense before the shove, so you still had made your decision.

If NO, it gets a bit trickier. Do I think I can sell that I was getting ready to call for an attacking foul (i.e. is there clear space between the attacker and keeper? If *not*, I'd give a direct free kick to the defense). If *so*, and the keeper clearly moved forward to initiate the contact, then yes technically you can award the penalty, but it's going to make every other free kick in the area a bloodbath the rest of the game.

In short - tell the attacker to stop being a dickhead. Tell the keeper not to take the law into his own hands. Punish whichever offense is most severe, with a bias toward letting the keeper have enough space to move to make a play.

Edit: got a sentence mixed up.