bash: adduser: command not found on centOS

Try adding /usr/sbin to your path.

I know the OP answer was solved, however for those who are running CentOS from within the docker container, which appears to be missing both adduser and useradd, the solution is to install shadow-utils (under root obviously):

yum install shadow-utils

/usr/sbin/useradd username -g groupname -d homedirectory