public void set(@Nullable String helper) {
      this.helper = helper;
    }
Default setter for Maven. Makes this syntax possible:

<pre>{@code
<configuration>
  <to>
    ...
    <credHelper>ecr-login</credHelper>
    ...
  </to>
</configuration>
}</pre>

@param helper the credential helper
jib-maven-plugin/src/main/java/com/google/cloud/tools/jib/maven/JibPluginConfiguration.java