title: Unable to find main class when running mvn sping-boot:build-image
body: <p>I've a maven multimodule project</p>
  <pre><code>Project
  |--- executable
  |       |--- src
  |       |--- pom.xml
  |--- core
  |       |--- src
  |       |--- pom.xml
  |--- pom.xml
  </code></pre>
  <p>The root pom.xml looks like this</p>
  <pre><code>&lt;parent&gt;
      &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
      &lt;artifactId&gt;spring-boot-starter-parent&lt;/artifactId&gt;
      &lt;version&gt;2.5.5&lt;/version&gt;
      &lt;relativePath/&gt; 
  &lt;/parent&gt;
  
  &lt;groupId&gt;com.playgrounds&lt;/groupId&gt;
  &lt;artifactId&gt;playgrounds-api&lt;/artifactId&gt;
  &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
  &lt;packaging&gt;pom&lt;/packaging&gt;
  
  &lt;properties&gt;
      &lt;java.version&gt;11&lt;/java.version&gt;
  &lt;/properties&gt;
  
  &lt;dependencies&gt;
  
      &lt;dependency&gt;
          &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
          &lt;artifactId&gt;spring-boot-starter-web&lt;/artifactId&gt;
      &lt;/dependency&gt;
  
      &lt;dependency&gt;
          &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
          &lt;artifactId&gt;spring-boot-starter-test&lt;/artifactId&gt;
          &lt;scope&gt;test&lt;/scope&gt;
      &lt;/dependency&gt;
  
      &lt;dependency&gt;
          &lt;groupId&gt;com.google.firebase&lt;/groupId&gt;
          &lt;artifactId&gt;firebase-admin&lt;/artifactId&gt;
          &lt;version&gt;8.0.1&lt;/version&gt;
      &lt;/dependency&gt;
  
  &lt;/dependencies&gt;
  
  &lt;modules&gt;
      &lt;module&gt;executable&lt;/module&gt;
      &lt;module&gt;core&lt;/module&gt;
  &lt;/modules&gt;
  </code></pre>
  <p>The exacutable module has a main class and the pom.xml looks like this</p>
  <pre><code>&lt;parent&gt;
      &lt;groupId&gt;com.playgrounds&lt;/groupId&gt;
      &lt;artifactId&gt;playgrounds-api&lt;/artifactId&gt;
      &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
      &lt;relativePath&gt;../&lt;/relativePath&gt;
  &lt;/parent&gt;
  
  &lt;artifactId&gt;executable&lt;/artifactId&gt;
  &lt;name&gt;executable&lt;/name&gt;
  &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
  &lt;packaging&gt;jar&lt;/packaging&gt;
  
  &lt;build&gt;
      &lt;plugins&gt;
          &lt;plugin&gt;
              &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
              &lt;artifactId&gt;spring-boot-maven-plugin&lt;/artifactId&gt;
          &lt;/plugin&gt;
      &lt;/plugins&gt;
  &lt;/build&gt;
  
  &lt;dependencies&gt;
      &lt;dependency&gt;
          &lt;groupId&gt;com.playgrounds&lt;/groupId&gt;
          &lt;artifactId&gt;core&lt;/artifactId&gt;
          &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
          &lt;scope&gt;compile&lt;/scope&gt;
      &lt;/dependency&gt;
      &lt;dependency&gt;
          &lt;groupId&gt;org.slf4j&lt;/groupId&gt;
          &lt;artifactId&gt;slf4j-simple&lt;/artifactId&gt;
          &lt;version&gt;2.0.0-alpha5&lt;/version&gt;
      &lt;/dependency&gt;
  &lt;/dependencies&gt;
  </code></pre>
  <p>The core module doesn't have a main class, and it looks like this.</p>
  <pre><code>&lt;parent&gt;
      &lt;groupId&gt;com.playgrounds&lt;/groupId&gt;
      &lt;artifactId&gt;playgrounds-api&lt;/artifactId&gt;
      &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
  &lt;/parent&gt;
  
  &lt;artifactId&gt;core&lt;/artifactId&gt;
  &lt;name&gt;core&lt;/name&gt;
  &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
  &lt;packaging&gt;jar&lt;/packaging&gt;
  </code></pre>
  <p>The command mvn clean package generates a SUCCESS output, however when I run the mvn spring-boot:build-image command, I get an error on the core module: The main class has not been found... Any ideas? I searched in the spring-boot documentation and I couldn't find what I'm missing.</p>