Unit Tests failing due to testcontainers

[vrann]

Environment details:

• Travis

Steps to reproduce the issue:

1. Create PR with no code change (i.e. documentation update) and wait for tests to get executed

Provide the expected results and outputs:

Unit tests passed successfully

Provide the actual results and outputs:

Unit Tests are failing with the following issue

org.testcontainers.containers.ContainerFetchException: Can't get Docker image: RemoteDockerImage(imageName=docker.elastic.co/elasticsearch/elasticsearch:6.7.2)
Caused by:
com.github.dockerjava.api.exception.NotFoundException: {"message":"No such image: alpine:3.5"}

Here is an example of the failing job: https://travis-ci.com/github/apache/openwhisk/jobs/500605165

Additional information you deem important:

• contacted Travis support team and here is what they responded:
  "It appears that the error Is because of a testcontainers bug. Is by any chance your library or code dependent on that? This issue was addressed in testcontainer since the release of version 1.15.0"

[ddragosd]

fixed on #5121.

Thanks @vrann !