title: Cannot build Docker image using spotify plugin
body: <p>I have been using spotify plug-in to build docker images, but it suddenly stops working for some reason, and it spews out error complaining about exec failure on spotify plug-in</p>
  
  <pre><code>[INFO] 
  [INFO] --- maven-jar-plugin:2.5:jar (default-jar) @ SimpleWebApp ---
  [INFO] Building jar: /home/test/opd_workspace/my_simple_webapp/target/SimpleWebApp-0.0.1-SNAPSHOT.jar
  [INFO] 
  [INFO] --- spring-boot-maven-plugin:1.3.5.RELEASE:repackage (default) @ SimpleWebApp ---
  [INFO] 
  [INFO] --- docker-maven-plugin:0.2.3:build (default-cli) @ SimpleWebApp ---
  SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
  SLF4J: Defaulting to no-operation (NOP) logger implementation
  SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
  [INFO] Copying /home/test/opd_workspace/my_simple_webapp/target/SimpleWebApp-0.0.1-SNAPSHOT.jar -&gt; /home/test/opd_workspace/my_simple_webapp/target/docker/SimpleWebApp-0.0.1-SNAPSHOT.jar
  [INFO] Copying src/main/docker/Dockerfile -&gt; /home/test/opd_workspace/my_simple_webapp/target/docker/Dockerfile
  [INFO] Building image imgprefix/SimpleWebApp
  [INFO] ------------------------------------------------------------------------
  [INFO] BUILD FAILURE
  [INFO] ------------------------------------------------------------------------
  [INFO] Total time: 5.282s
  [INFO] Finished at: Wed Jun 01 19:42:14 EDT 2016
  [INFO] Final Memory: 27M/340M
  [INFO] ------------------------------------------------------------------------
  [ERROR] Failed to execute goal com.spotify:docker-maven-plugin:0.2.3:build (default-cli) on project SimpleWebApp: Exception caught: Request error: POST unix://localhost:80/v1.12/build?t=imgprefix/SimpleWebApp: 500: HTTP 500 Internal Server Error -&gt; [Help 1]
  </code></pre>
  
  <p>My pom.xml plug-in is very simple, and maven project build and package is ok</p>
  
  <pre><code>&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;plugin&gt;
                  &lt;groupId&gt;com.spotify&lt;/groupId&gt;
                  &lt;artifactId&gt;docker-maven-plugin&lt;/artifactId&gt;
                  &lt;version&gt;0.2.3&lt;/version&gt;
                  &lt;configuration&gt;
                      &lt;imageName&gt;${docker.image.prefix}/${project.artifactId}&lt;/imageName&gt;
                      &lt;dockerDirectory&gt;src/main/docker&lt;/dockerDirectory&gt;
                      &lt;resources&gt;
                          &lt;resource&gt;
                              &lt;targetPath&gt;/&lt;/targetPath&gt;
                              &lt;directory&gt;${project.build.directory}&lt;/directory&gt;
                              &lt;include&gt;${project.build.finalName}.jar&lt;/include&gt;
                          &lt;/resource&gt;
                      &lt;/resources&gt;
                  &lt;/configuration&gt;
              &lt;/plugin&gt;
          &lt;/plugins&gt;
  </code></pre>