It would be nice if environment variables (or another placeholder replacement mechanism) could be used for the config files in /usr/share/jenkins/ref/. For example:

hudson.tasks.Mailer.xml:

<?xml version='1.0' encoding='UTF-8'?>
<hudson.tasks.Mailer_-DescriptorImpl plugin="mailer@1.19">
  <hudsonUrl>http://jenkins.example.com/</hudsonUrl>
  <smtpHost>smtp.example.com</smtpHost>
  <useSsl>false</useSsl>
  <charset>UTF-8</charset>
</hudson.tasks.Mailer_-DescriptorImpl>

jenkins.model.JenkinsLocationConfiguration.xml:

<?xml version='1.0' encoding='UTF-8'?>
<jenkins.model.JenkinsLocationConfiguration>
  <adminAddress>no-reply-jenkins@example.com</adminAddress>
  <jenkinsUrl>http://jenkins.example.com/</jenkinsUrl>
</jenkins.model.JenkinsLocationConfiguration>

I would like to provide these default configuration files in /usr/share/jenkins/ref/, but the http://jenkins.example.com/ should be specified dynamically when running the container (e.g. for another instance in a test environment).

So I'd like to be able to write something like
<jenkinsUrl>${JENKINS_URL}</jenkinsUrl>
or maybe with a default value
<jenkinsUrl>${JENKINS_URL-http://jenkins.example.com/}</jenkinsUrl>

and then specify a value at runtime with
docker run -e "JENKINS_URL=http://jenkins-test.example.com/"

What do you think?