Support clusters that can't see GitLab
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### Release notes
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### Problem to solve
As a self-managed GitLab user, I need to deploy to my in-cloud clusters that can not access GitLab directly. On the other hand, GitLab can access these clusters.
### Proposal
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Document how to do the following
* Connect private network of the GitLab instance to the cloud network of the Kubernetes cluster. E.g. for AWS see [Connect your VPC to remote networks](https://docs.aws.amazon.com/vpc/latest/userguide/vpn-connections.html) and more specifically [What is AWS Direct Connect?](https://docs.aws.amazon.com/directconnect/latest/UserGuide/Welcome.html).
* Expose a reverse proxy for kas to the internet like we do for GitLab.com. For extra security the proxy can be firewalled to only accept traffic from IPs used for egress from the Kubernetes cluster.
### Intended users
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* [Cameron (Compliance Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#cameron-compliance-manager)
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* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* [Rachel (Release Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#rachel-release-manager)
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### Metrics
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### References
- https://gitlab.com/groups/gitlab-org/configure/-/epics/8+
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