Propose to add config option 'transformableAllowList'

docs/design_proposals/configurable-transformable-allowlist.md

@@ -0,0 +1,74 @@
+# Configurable transformableAllowList
+
+* Author(s): Ke Zhu (@shawnzhu)
+* Design Shepherd: Tejal Desai (tejal29)
+* Date: 2021-07-16
+* Status: Draft
+
+## Objectives
+
+Configurable transformableAllowList for transforming manifests.
+
+## Background
+
+Skaffold can only transform manifests from a non-extensible allowlist. When
+using any CRD out of this allowlist, skaffold can not transform it.
+
+Open issues concerning this problem:
+
+* Image not recognized in crd k8s manifest ([#4081](https://github.com/GoogleContainerTools/skaffold/issues/4081))
+
+There was comments out of the above issue to make this allowlist extensible.
+
+The goal of this document is to create an agreement on the configuration option
+and specification to extend `skaffold.kubernetes.manifest.transformableAllowList`.
+
+## Design
+
+Add configuration `deploy.config.transformableAllowList` in `skaffold.yaml`:
+
+Notice that any new configuration option will be appended to existing allowlist. 
+
+### Detailed discussion
+
+Option in `skaffold.yaml`
+
+```YAML
+deploy:
+  config:
+    transformableAllowList:
+    - Group: example.com
+      Kind: Application
+    - Group: argoproj.io
+      Kind: Workflow
+    - Group: tekton.dev
+      Kind: Task
+```
+
+## Open issues/Questions
+
+Since it is an allowlist, neither options could disable transformation on any
+built-in resource like `ReplicaSet` or `Deployment`.
+
+Is there any need to work out a deny list?
+
+## Implementation plan
+
+1. `pkg/skaffold/schema/latest/v1/config.go` - Add config option
+`transformableAllowList` to `DeployConfig`.
+2. `pkg/skaffold/kubernetes/manifest/visitor.go` - Add new parameter `transformableAllowList` 
+to `*ManifestList.Visit()` by appending it to existing coded `transformableAllowList`
+3. `pkg/skaffold/kubernetes/manifest/images.go` - Add new parameter to `*ManifestList.ReplaceImages()`
+to support given `transformableAllowList`
+4. Instrument each deployer to use the new parameter `transformableAllowList`
+
+## Integration test plan
+
+Please describe what new test cases you are going to consider.
+
+1.  Unit and integration tests for `visitor.go`.
+
+    The integration tests should be written to catch situations such as this
+    configurable allowlist is either empty or empty array.
+
+3.  Document this new configuration option