Allow to get a TriggerBuild instance from DockerHubImage

Summary:
Goal is to allow fluent pattern:

$image = new DockerHubImage("acme", "foo");
$payload = $image
    ->getTriggerBuild('0000')
    ->getPayloadForDockerTag('7.1')

Ref T965.

Test Plan: New unit test.

Reviewers: dereckson

Tags: TDD

Maniphest Tasks: T965

Differential Revision: https://devcentral.nasqueron.org/D598