fix usage info for tools/build-image.sh
fix usage info for tools/build-image.sh

parasgithub/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,jakebailey/PrairieLearn,mwest1066/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,jakebailey/PrairieLearn,rbessick5/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,rbessick5/PrairieLearn,jakebailey/PrairieLearn,mwest1066/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,jakebailey/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn