Add the ability to configure the network used by Docker fixtures using DOCKER_FIXTURES_NETWORK

[raul-arabaolaza]

To pick jenkinsci/docker-fixtures#32 basically.So I can use docker-compose to launch the ATH in a trivially portable way.

This pr should be enough to test the change, I have manually tested this and on my personal CI system

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[raul-arabaolaza]

There are 0 new tests failing, 70 existing failing and 72 skipped.

It is a bit dificult to compare with master build as it does not seem to have coherent results, however last master result is 50 failed and other PRs also have 50 failed, so I will review in depth the tests to be sure

[raul-arabaolaza]

This is what I am gonna do to properly check this.

• I will remove the dependency update and let the pr builder to run with just the README update this will give me an accurate baseline
• Then I will add again the dependency update and compare the results with the previous execution
• After that thoughts and prayers 😁

[raul-arabaolaza]

Ok, so https://ci.jenkins.io/job/Core/job/acceptance-test-harness/view/change-requests/job/PR-655/ should be my baseline once it finishes

[raul-arabaolaza]

Using https://ci.jenkins.io/blue/organizations/jenkins/Core%2Facceptance-test-harness/detail/PR-655/3/tests as baseline, I have restored the fixtures update, will see

[raul-arabaolaza]

OK, almost same results but one test failing building docker images, does not seem related, so I will retrigger

[raul-arabaolaza]

Retriggering solved the unexpected failures but gave a different number of tests, at this point I am convinced I am suffering flakiness and none of the failures are related to my change