title: com.spotify/dockerfile-maven-plugin: tag failed: path is 'null'
body: <p>For a multimodule maven project build using com.spotify:dockerfile-maven-plugin on a CI runner, building the docker image with <code>dockerfile:build dockerfile:tag</code> in a previous step, and running <code>mvn clean deploy dockerfile:push</code> in deploy step results in an error:</p>
  
  <pre><code>Failed to execute goal com.spotify:dockerfile-maven-
  plugin:VERSION:tag (default-cli) on project PROJECT_NAME: Execution 
  default-cli of goal com.spotify:dockerfile-maven-plugin:VERSION:tag 
  failed: path is 'null'.
  </code></pre>
  
  <p>Logs:</p>
  
  <p><code>Tagging image null as REPO_NAME/IMAGE_NAME</code></p>
  
  <p>Evidently, the built image isn't getting tagged properly. 
  <code>pom.xml</code>:</p>
  
  <pre><code>                &lt;configuration&gt;
                      &lt;repository&gt;${prefix}/${project.artifactId}&lt;/repository&gt;
                      &lt;tag&gt;${tag}&lt;/tag&gt;
                      &lt;buildArgs&gt;
                        &lt;JAR_FILE&gt;${project.build.finalName}.jar&lt;/JAR_FILE&gt;
                      &lt;/buildArgs&gt;
                      &lt;skip&gt;false&lt;/skip&gt;
                  &lt;/configuration&gt;
  
  </code></pre>
  
  <p>But I can't really find what I'm doing wrong.</p>