Hi Rj,
The message "xnat-web_1 | Postgres is unavailable - sleeping" is normal, because the xnat-web container waits for postgres to fully start before starting its Tomcat process. The "xnat-web_1 | psql: could not translate host name "xnat-db" to address: Name does not resolve" message, however indicates that the postgres container either hasn't been defined or can't be found on the default Docker network.
Assuming you have no data loaded into the system, I would try:
ensure all containers are stoped (docker ps -> docker kill CONTAINER_ID),
and clone the above repo to an empty local directory owned by your user.
$ docker-compose up
Let me know how that works.
Thanks,
Matt