K8s versions used for e2e

[ritazh]

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Currently we default to K8s v1.19.0 for the e2es.

gatekeeper/Makefile

Line 13 in 106494f

KUBERNETES_VERSION ?= v1.19.0

When a change is introduced that needs to be validated across older K8s versions (for back compat) and newer K8s versions, we need to build confidence that the change introduced continues to work in older K8s versions and new versions. e.g. removal of selfLink in K8s v1.20

Few things to consider:

• Do we run a matrix of all supported K8s versions or do we always use the latest?
• If we do want to run the tests against a matrix, which versions do we use? (e.g. currently supported K8s versions: 1.18, 1.19, 1.20)
• If we do want to run the tests against a matrix, do we do this as a PR gate or only against master or a job we can trigger?

Anything else you would like to add:
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Environment:

• Gatekeeper version:
• Kubernetes version: (use kubectl version):

[maxsmythe]

Is there anything preventing us from running 5 of these jobs in parallel? Running these tests against multiple cluster versions could be essentially free, with the main cost being that flakiness will hurt twice as much.

How many dimensions would be in the matrix? FWIW I don't think we need to be testing manifest + helm for all K8s versions.