Allow specifying network for instance test runner

Lucas Lirequested to merge
lucas/instance-network into master

What does this MR do and why?

Describe in detail what your merge request does and why.

This adds support for specifying a Docker network for instance tests.

In my case, working on https://gitlab.com/groups/gitlab-org/distribution/build-architecture/-/epics/34+, QA instances are deployed in GKE, and SSH is exposed via an HAProxy layer — so accessing Git over SSH requires a custom GIT_SSH_COMMAND.

Since passing extra environment variables isn't currently supported, a workaround is to run a proxy container in the QA runner’s Docker network and point the QA job at the proxy’s DNS address.

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Edited by Lucas Li

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