title: How to set base image for DockerPlugin of sbt native packager?
body: <p>I want to dockerify a scala application.</p>
  
  <p>I use the <code>sbt-native-packager</code>, and when building the image locally via</p>
  
  <pre><code>sbt docker:publishLocal
  </code></pre>
  
  <p>I was surprised to realize that it would use:</p>
  
  <pre><code>openjdk:latest
  </code></pre>
  
  <p>as its base image.</p>
  
  <p>I like to point my docker image against certain version, and I know that my application currently require Java 8 and will crash on Java 9; and the <code>latest</code> tag might point to a new major version in the future.</p>
  
  <p>I am a bit confused on how the Docker plugin works through sbt-native-packager. Why is it using <code>openjdk:latest</code> as its base? How can I set it to something different, e.g.: <code>openjdk/8-jdk</code>?</p>