Within the context of a running container, the docker module supports arbitrary commands such as:
docker::exec { 'cron_allow_root':
detach => true,
container => 'mycontainer',
command => '/bin/echo root >> /usr/lib/cron/cron.allow',
onlyif => 'running',
tty => true,
env => ['FOO=BAR', 'FOO2=BAR2'],
unless => 'grep root /usr/lib/cron/cron.allow 2>/dev/null',
refreshonly => true,
}