[bitnami/common] Digest/Tag new approach backward compatible

[carrodher]

Current behavior

1.- Default values

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
   digest: ""
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq:3.10.7-debian-11-r4

2.- Setting the image.digest

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
   digest: "sha256:aaa"
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq@sha256:aaa

3.- Removing the image.digest (old default)

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq@<nil>

Behavior with changes in this PR

1.- Default values

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
   digest: ""
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq:3.10.7-debian-11-r4

2.- Setting the image.digest

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
   digest: "sha256:bbb"
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq@sha256:bbb

3.- Removing the image.digest (old default)

$ cat values.yaml
 image:
   registry: docker.io
   repository: bitnami/rabbitmq
   tag: 3.10.7-debian-11-r4
$ helm template . | grep 'image:'
          image: docker.io/bitnami/rabbitmq:3.10.7-debian-11-r4

Note the difference in the third case. With the current approach (bitnami/common@2.0.0), when image.digest doesn't exist in the values.yaml it returns image: docker.io/bitnami/rabbitmq@<nil>. With the changes in this PR (bitnami/common@2.0.1), when the image.digest doesn't exist in the values.yaml a valid image is returned image: docker.io/bitnami/rabbitmq:3.10.7-debian-11-r4 using the image.tag as usual.

This is what it's behind the issue described at #12026

Once merged this PR, we should release a new version of the different Helm charts bundling bitnami/common@2.0.1

[jouve]

glad to see you reintroduced parts of #11844 :)