The Docker setup instructions can include a little text about proper starting folder before the docker run command.
Example. Say you cloned to folder tddec-code, and you are in folder ~/github,. Then the ttd code is in folder ~/github/tddec-code. While you are in ~/github execute:
WORKING_DIR=/usr/src/tddec-code
export CPPUTEST_HOME=$WORKING_DIR/cpputest
docker run -it -v $MOUNT_DIR -w $WORKING_DIR -e CPPUTEST_HOME gcc:7 make