Can't use yum inside Docker container running on CentOS

I am unable to run any yum commands inside my Docker container without getting the following error:

ovl: Error while doing RPMdb copy-up:
[Errno 13] Permission denied: '/var/lib/rpm/Group'
You need to be root to perform this command.

I'm confused because I'm pretty sure docker containers are run with the default user root. Still, if I try putting sudo in front of a yum install -y <package> or yum update -y command I get:

/bin/sh: sudo: command not found

I'm using the following base image so I can easily run a Java Wildfly application inside Docker: https://hub.docker.com/r/jboss/wildfly/

The underlying distro is CentOS Linux release 7.2.1511 (Core)

Turns out the user was set to jboss in the base image.
When is switched to user root with the dockerfile command USER root everything worked.