title: Spring Boot not loading system env vars into application.properties
body: <p>I have exported some simple db creds on my machine as ENV variables.</p>
  
  <pre><code>export MYSQL_USER=root
  export MYSQL_PASS=pass
  </code></pre>
  
  <p>I'm trying to load these dynamically into my profile into spring boot, but to no avail. I'm starting to believe this is because the application runs inside a TomCat container and therefore isolated from the machine env vars. </p>
  
  <h1>Development Environment Application Properties File</h1>
  
  <pre><code>spring.datasource.username = ${MYSQL_USER}
  </code></pre>
  
  <p>However, it's still being seen as literal plain text of <code>${MYSQL_USER}</code>. Does Spring Boot have the capability for readingg in system environment variables?</p>