Create shared Geo instance for test failure debugging

Problem to solve

When investigating failing Geo end-to-end tests, it would be more representative to setup the Geo nodes (using Docker images) and run the tests in the same manner used during CI, than to use GDK.

Proposal

Create a shared GCP Geo instance in a similar manner described in gitlab-qa/lib/gitlab/qa/scenario/test/integration/geo.rb and gitlab/qa/qa/ee/scenario/test/geo.rb. The setup could consist of three Docker containers (primary node, secondary node, and third to execute the tests) and would include scripts/software to help with debugging (e.g. Bash).

During a recent Geo test failure debug session, this ran well on a Linux host with ~16 GB RAM.