After upgrading jib-maven-plugin from 3.0.0 to 3.1.0 entrypoint=INHERIT doesn't ignore mainClass

[estambakio-sc]

Environment:

• Jib version: 3.1.0 (jib-maven-plugin)
• Build tool: Maven 3.6.3
• OS: Linux

Description of the issue:

After updating jib-maven-plugin from version 3.0.0 to version 3.1.0 execution of dockerBuild goal for container with container.entrypoint=INHERIT configuration option fails with error:

[ERROR] Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.1.0:dockerBuild (docker-image-package) on project javadocs: Main class was not found, perhaps you should add a `mainClass` configuration to jib-maven-plugin -> [Help 1]

README says that entrypoint=INHERIT means mainClass is not taken into account, which seems to be broken in 3.1.0

Expected behavior:

After updating 3.0.0 -> 3.1.0 build should succeed because release notes don't say anything about changed behaviour of entrypoint=INHERIT configuration.

Steps to reproduce:

1. Add jib-maven-plugin configuration from block below with version 3.0.0 into pom.xml:

2. Execute the following command: mvn package and Docker image is built successfully.

3. Change jib-maven-plugin version to 3.1.0, execute mvn package - build fails with the following error:

[ERROR] Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.1.0:dockerBuild (docker-image-package) on project javadocs: Main class was not found, perhaps you should add a `mainClass` configuration to jib-maven-plugin -> [Help 1]

jib-maven-plugin Configuration:

<build>
    <plugins>
      <plugin>
          <groupId>com.google.cloud.tools</groupId>
          <artifactId>jib-maven-plugin</artifactId>
          <version>3.0.0</version> <!-- 3.0.0 works, 3.1.0 fails -->
          <configuration>
            <from>
              <image>nginx:alpine</image>
            </from>
            <to>
              <image>a:b</image>
            </to>
            <container>
              <entrypoint>INHERIT</entrypoint>
            </container>
          </configuration>
          <executions>
            <execution>
              <id>docker-image-package</id>
              <phase>package</phase>
              <goals>
                <goal>dockerBuild</goal>
              </goals>
            </execution>
          </executions>
        </plugin>
    </plugins>
</build>

Log output:

[ERROR] Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.1.0:dockerBuild (docker-image-package) on project javadocs: Main class was not found, perhaps you should add a `mainClass` configuration to jib-maven-plugin -> [Help 1]

Additional Information:

[chanseokoh]

Will fix this and make a patch release.

[chanseokoh]

@estambakio-sc @dkuzmianok-sc just released Jib 3.1.1 with the fix.