title: Use environment variable in YAML with Gradle
body: <p>I am working in a Spring Boot project with Gradle and an application.yml with the properties.</p>
  
  <p>How can I declare the property logging.config in a YML file to config Logback using an environment variable?</p>
  
  <p><code>logging:
    config: ${PATH}/application-log.xml
  </code></p>
  
  <p>This way, the build fails in the :processResources task. I understand, Gradle tries to replace ${...}</p>
  
  <p>I have tried changing to ${'PATH'} but I got the error:</p>
  
  <p><code>Exception in thread "main" java.lang.IllegalArgumentException: Could not resolve placeholder ''PATH'' in string value "${'PATH'}/application-log.xml"
      at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:174)
  </code></p>
  
  <p>Thanks in advance</p>