(maint) Parameterize additional Docker build opts
(maint) Parameterize additional Docker build opts

 - Use an environment variable so the custom changes are more visible near the
   top of the build

puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server