Parameterize the image used for init containers.

[ksuderman]

Most images used can be specified, and overridden, in the values.yaml file. Except the images used in the init containers in the jobs_init.yaml template.

This PR parameterizes those images (four in total) as well. The only issue to be settled is where in the values.yaml file these should be specified. For now I have added them under the jobs block:

#- Additional dynamic rules to map into the container.
jobs:
  init:
    #- The Docker image to use for the init containers
    image:
      repository: alpine
      tag: 3.16
      pullPolicy: IfNotPresent

Closes #499

[nuwang]

lgtm!