How to install drywall over electrical box connected to rigid conduit?
On the metal box remove the ears, put up the drywall, then pull the box out sightly past the wall and reattach the ears.

If, in addition to screws in the "ears", it is necessary to to provide additional clamping of the box on the inside of the drywall, assuming the bottom of the box has no cables or conduit in the way, use the flat metal folding clips to hold the box to the inside of the drywall. Raco old work box supports. One drawback to the use of these metal box supports is that the end folded inside the box can touch the hot screw contacts. So if they are used, a good practice is to use electrical tape around the receptacle to cover the screw contacts. I suppose one could instead put tape on the part of the box support that is folded inside the box on the hot side. In this case the metal receptacle box is apparently mounted horizontally so the receptacle could be placed with the hot side up and away from the folded support.  

But a neater way to securely hold the box on the inside of the drywall would be to adhere wood strips (such as 1/4" or 1/2" thick plywood) on the insides of the drywall opposite the ears on the box. This would give something for screws in the holes in the ears to bite into. If non-combustible materials are required, then metal flashing could be cut and secured to the drywall before it is screwed place to the wall. Then drill through the holes in the ears through the drywall, and through the metal for screws to hold the ears.