For this particular rebuild you needed to download a new base image, shown from the WARNING to Status lines in your screenshot. This is large, 2.81GB according to my system, so what you’re seeing is the space lost to that new version of the base image.
# docker images -a
REPOSITORY TAG IMAGE ID CREATED SIZE
discourse/base latest ab71a5d97460 4 days ago 2.81GB
Using ./launcher cleanup will clear unused containers and images which may reclaim enough space and as @pfaffman said, pruning with docker directly may be able to clear further unused objects, though I personally haven’t seen any significant benefit beyond the containers and images.
Another thing that may help is to clear the apt cache which can become quite large.
apt-get clean