[promtail] imagepullsecret for private repos

[rayotte]

Is your feature request related to a problem? Please describe.
The loki helm chart has an option in the values.yaml file to use image pull secrets in order to pull the image from a private repository:

 image:
  repository: grafana/loki
  tag: 1.5.0
  pullPolicy: IfNotPresent

  ## Optionally specify an array of imagePullSecrets.
  ## Secrets must be manually created in the namespace.
  ## ref: https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/
  ##
  # pullSecrets:
  #   - myRegistryKeySecretName

This config doesn't appear to be in the promtail chart values.yaml file. Is there a way to do this with the promtail chart as well or is it possible to have it added in to work the same way it does in the loki chart?

Additional context
Tried adding it to the promtail serviceaccount as well but that doesn't seem to support that either