Hardcoded initContainer image name

[krisko]

Please don't hardcode image names into helm chart.
I use custom postgresql image, because bitnami doesn't support armv8.

values.yaml:

postgresql:
  enabled: true
  image:
    registry: docker.io
    repository: postgres
    tag: 11.1

Helm chart should respect custom image values.

Introduced in: 8c75469

[Couto]

Just to point out, and hopefully raise the visibility of this problem, this is related to #94