Allow not creating service accounts (serviceAccount.create=false)

[kyanar]

Currently the chart forces you to allow it to create the service account, which is something we have a general rule not to allow vendored charts to do (our practice is to pre-create the namespaces, service accounts, and cluster role bindings using a common Helm chart).

The Buildkite agent Helm chart should allow disabling creation of the service account, and allowing specification of the pre-created one.

[rajat2911]

Hello @rimusz do we have any plans to disable the service account ?

[rimusz]

@rajat2911 not in my plans, but you are free to create the PR for.