Be aware that the prebuilt Docker image is known to have issues on non-Intel macOS machines. It will work correctly if the base ACL2 Docker image is rebuilt from scratch on an M1/M2 machine, and the ACL2s image is built on top of this new image.
By default, certification is done with 4 parallel tasks. This can be changed by overriding the ACL2_CERTIFY_OPTS build argument. (i.e. using docker build --build-arg ACL2_CERTIFY_OPTS="-j 2")
To update the Gradescope Docker image, you first need to update the corresponding acl2-docker image. Then, update the IMAGE_VERSION value in make-gradescope.sh to match whatever was used in the acl2-docker's make-gradescope.sh script.