WAR containerization broken with the base image change

[chanseokoh]

Glad we had a tomcat integration test and were able to catch this before making it out.

In #3124, we were computing the entrypoint while mistakenly assuming that the base image is always the default Jetty image.

com.google.cloud.tools.jib.gradle.WarProjectIntegrationTest > testBuild_tomcatServlet25 FAILED
    java.lang.AssertionError: expected:<Hello world> but was:<null>
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.failNotEquals(Assert.java:835)
        at org.junit.Assert.assertEquals(Assert.java:120)
        at org.junit.Assert.assertEquals(Assert.java:146)
        at com.google.cloud.tools.jib.gradle.WarProjectIntegrationTest.verifyBuildAndRun(WarProjectIntegrationTest.java:63)
        at com.google.cloud.tools.jib.gradle.WarProjectIntegrationTest.testBuild_tomcatServlet25(WarProjectIntegrationTest.java:52)

    java.lang.RuntimeException: Command 'docker stop ac3948ea573d7b08e5e320ad02a5772aeb5d1f58491cee99a340d914860da171' failed: Error response from daemon: No such container: ac3948ea573d7b08e5e320ad02a5772aeb5d1f58491cee99a340d914860da171
        at com.google.cloud.tools.jib.Command.run(Command.java:79)
        at com.google.cloud.tools.jib.Command.run(Command.java:52)
        at com.google.cloud.tools.jib.gradle.WarProjectIntegrationTest.tearDown(WarProjectIntegrationTest.java:40)