Simplify Cluster Image Scanning Setup
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### Release notes
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### Problem to solve
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Currently, users need to maintain a list of namespaces (and other resources) to scan in their GitLab Kubernetes Agent `config.yaml` file. Users also need to maintain a list of namespaces to scan as part of their Security Policy. Additionally, the list of namespaces that are present in a cluster can frequently change. This creates a lot of overhead for users who need to try to keep all three areas in sync with each other.
### Intended users
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* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
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* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* [Alex (Security Operations Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#alex-security-operations-engineer)
### User experience goal
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### Proposal
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1. Users will no longer need to configure which resources are scanned in their GitLab Kubernetes Agent `config.yaml` file.
### Further details
Engineering DRI:
- ~backend @bwill
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The **proposal** requirements are intentionally vague to allow engineering freedom to design the solution. Some ideas of potential ways to implement this are below. This list is not intended to limit the implementation approach and is rather shared to spark ideas on how this might be accomplished.
#### Idea 1
1. Upon a request from the agent to retrieve the config.yaml file, we search GitLab to identify all of the security policy projects that had at least one cluster image scan configured for the specified Kubernetes cluster.
1. We then collect a list of all namespaces that are to be scanned and send that list back to the Kubernetes agent that requested it.
1. This whole process would be transparent to the end user (although, I imagine that there may be some performance considerations given that we are not yet in the database).
#### Idea 2
1. When a user edits a Cluster Image Scanning security policy, we then store a list of namespaces to be scanned in the database (or elsewhere) and associate that list with each Kubernetes Agent in the policy. We manage and update this list each time a Cluster Image Scanning policy is created, edited, or deleted.
1. When a Kubernetes Agent checks in with GitLab, we are able to easily lookup the list of namespaces to be scanned and return it. This approach may overcome some of the complex lookups and potential performance issues associated with Idea 1.
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### Is this a cross-stage feature?
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