In this video you can see three versions of dynamic knee raises that that can be used after goalkeepers are properly warmed up and prepared for dynamic movements such as these.
Flat cones on the floor are placed in certain order (but they could also be placed randomly, of course), and goalkeepers should raise the knee with the same side leg when they get to certain cone.
In option 1 of the exercise, the knee raises are in front of the body.
In option 2 the knee raises are laterally/sideways.
In option 3 every second knee raise is going to alternate and be once in front of the boy, next time laterally/sideways.