Native read-only parameter for docker run

[jacksgt]

Quite often, I'm finding myself having to pass --read-only as an extra_parameter to ::docker::run, like so:

 ::docker::run { 'postgres':
    image            => "puppet/puppetdb-postgres:${version}",
    ...
    extra_parameters => [
      '--read-only=true',
    ]
}

It would be great to have a 'native' parameter for this, i.e. read_only => true.