Make gitlabci configuration compatible with docker test runners

Problem/Motivation

Discovered on 📌 Use a dedicated runner for performance tests RTBC . This is low priority but it might become higher priority if we start to get donated test runners.

Steps to reproduce

Set up a custom gitlab runner using docker instead of kubernetes, try to run chromedriver tests, they will fail. This is because 'localhost' doesn't work on docker, or at least, it only works in some contexts and not others.

Proposed resolution

There are at least three changes required (See the performance job changes in that MR).

1. chromedriver needs to be referenced as chrome:9515 not localhost:9515

2. At least for chromedriver, SIMPLETEST_BASE_URL needs to rely on the $HOSTNAME environment variable instead of 'localhost' - however I don't know if that would continue to work for kubernetes.

3. Chromedriver needs -allowed-origins=* to accept connections from another docker container.

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet