Configure base images

[aranega]

Closes CH-287

Implemented solution

The solution relies on the addition of an extra entry in the value-template.yaml file which allows the user to declare a mapping to change the base image. The mapping affects the build args which are passed to skaffold to build the images, effectively replacing the base image with the one declared in the value-template.yaml file. The PR also document about the path to override for apps wich do not rely on an accessible Dockerfile (e.g: argo, or the gatekeeper)

Note: Some tests realted to secure secret are failing on develop the same pre-existing failing tests are still failing with this PR.

How to test this PR

To test the PR, the value-template.yaml file can be updated with a specific mapping for the source image, e.g:

source_images:
  MY_BASE_IMAGE: "mybaseimg:123"

and the new deployment configuration generated using harness-deployment.
After the generation, the skaffold.yaml file can be opened to see that buildArgs have been impacted for myapp and that the new base image mybaseimg:123 have been properly referenced.

Sanity checks:

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• The issue is well described: clearly states the problem and the general proposed solution(s)
• In this PR it is explicitly stated how to test the current change
• The issue seta the scope and the type of issue (bug, story, task, etc.)
• The relevant components are indicated in the issue (if any)
• All the automated test checks are passing
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Breaking changes (select one):

• The present changes do not change the preexisting api in any way
• This PR and the issue are tagged as a breaking-change and the migration procedure is well described above

Possible deployment updates issues (select one):

• There is no reason why deployments based on CloudHarness may break after the current update
• This PR and the issue are tagged as alert:deployment

Test coverage (select one):

• Tests for the relevant cases are included in this pr
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Documentation (select one):

• The documentation has been updated to match the current changes
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[filippomc]

I'm afraid the configuration is more complicated that it needs to be, as this is supporting the reuse of the same name in different context, which is unnecessary.

I think it's better to use one name for each image that is used, and make it possible to override it, regardless of where it's used.

For instance, cloudharness-frontend-build needs a node image. I'd do FROM $BASE_NODE and then

image_sources:
  BASE_NODE: node: 22
  GATEKEEPER: quay.io/gogatekeeper: 5.0.0
  KEYCLOAK: quay.io/keycloak: 26.0.0
  ...

We can pass all args to all builds then