feat(charts/dex): add initContainers

[bdomars]

Overview

Adding an option to specify initContainers via values.

What this PR does / why we need it

The use case I have for this is to be able to run a custom built image in an initContainer that will copy a custom theme into an emptyDir volume that is mounted by both the Dex container and the initContainer.

I've been doing something like this

initContainers:
- image: dex-custom-theme:v1
  imagePullPolicy: Always
  name: install-theme
  resources: {}
  volumeMounts:
  - mountPath: /custom-theme
    name: custom-theme
volumes:
- emptyDir: {}
  name: custom-theme
volumeMounts:
- mountPath: /custom-theme
  name: custom-theme
env:
  DEX_FRONTEND_DIR: /custom-theme

This allows to use a custom theme for Dex with the helm chart and without a need for rebuilding the Dex image. This simplifies automatic updates with things like Renovate.

Checklist

• Change log updated in Chart.yaml (see the contributing guide for details)
• Chart version bumped in Chart.yaml (see the contributing guide for details)
• Documentation regenerated by running make docs