The Dockerfile for the official mongo image is here. The default command is mongod but you can override to add the --auth switch assuming user's are already configured.
docker run -d .... mongodb:latest mongod --auth
If the user has to be created then you need to volume mount a startup script into /entrypoint.sh
to replace the default startup script and then have that script create users and start mongo with the auth switch.
docker run -d .... -v $PWD/my_custom_script.sh:/entrypoint.sh mongodb:latest