Locate a hardware store where you can obtain a wall protector. They are large circular plastic self adhesive plastic pieces that will adhere to virtually any wall.
Gently open the door and locate exactly where the doorknob touches against your interior wall. You can make a mark with a pencil if you desire.
Peel the adhesive backing off the wall protector.
Press the protector onto the wall where it will be contacted by your doorknob. The foam adhesive on the back of the protector absorbs shock.
Paint the protector to match the color of your wall, if you so desire. If you have wallpaper, you can also glue a bit of wallpaper over the bumper as well.