Updates to E2Es to allow an offline container registry

What does this MR do and why?

These changes are being made to support some planned work in gitlab-qa to improve the setup of the airgapped/offline-environment scenario. Draft: Add an offline container registry for Of... (gitlab-qa!1102)

• introduce a new ENV to allow a configurable the host name for the container registry for docker images

  • the proposed update to airgapped/offline-environment testing is to store any required images in a dockerized registry which can be attached to the airgapped test environment
  • we'll need the ability to target either the default registry.gitlab.com or this testregistry.airgapped endpoint and an ENV makes this configuration possible

• modify the behaviour of the 'prove_airgap' check

  • in the testing the changes to the airgapped scenario, noted that because we're using the 'docker network create --internal' to create an airgapped network we don't have the ability to even determine what the IP address in this check is. This causes as an exception to be raised which is now handled.

How to set up and validate locally

Behaviour should remain as before.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.