You can use Puppet commands, such as resource.
The following command lists all of the packages installed on the image.
docker run puppet/puppet-agent-ubuntu resource package --param provider
To find out about the packages installed on the host, rather than in the container, mount in various folder from the host like so.
docker run --privileged -v /tmp:/tmp --net host -v /etc:/etc -v /var:/var -v /usr:/usr -v lib64:/lib64 puppet/puppet-agent-ubuntu resource package
This will do!