Rollout Network Per-Build for Linux shared runners

Status

• Rollout FF to the private runners manager
• Rollout FF to the shared-gitlab-org runners manager
• Rollout FF to the shared runners manager.
• Rollout FF to the saas-linux-... roles.

Overview

In gitlab-org/gitlab-runner!1569 (merged) we added a new networking feature for the Docker executor where it creates a docker network per build, more details here. This is behind a feature flag called FF_NETWORK_PER_BUILD. We need to roll this out to production to make sure it works at the scale of GitLab.com before we enable it by default in https://gitlab.com/gitlab-org/gitlab-runner/-/issues/11751.

Proposal

1. We should update our chef configuration to enable the feature flag.
2. The feature flag should be enabled gradually, meaning private first, then shared-gitlab-org, last runners-manager-shared and saas-linux-... roles.
3. Make sure for each change there is a change management issue