rename helm `imageKey` to `image` to match starter workflows

[davidgamero]

Description

The recommended generated key names in draft don't line up with the keys in the starter workflows, so this PR changes the imageKey key to just image so that they are all consistent

This is a breaking change for consumers of the IMAGENAME variable, as it will no longer can contain both an image name and tag.

Previously, alpine:latest was a valid IMAGENAME, but now the IMAGETAG variable (which defaults to "latest" following docker and kubernetes convention) should be used to pass a tag.

Usages of IMAGENAME that include a tag will now need to use the IMAGETAG variable for passing a tag, as passing an image name like alpine:latest will result in double-tagging such as alpine:latest:latest

• [x] Breaking Change - new variable IMAGETAG and changed usage of IMAGENAME
• [x] Bug fix (non-breaking change which fixes an issue)
• [x] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration. Is it a breaking change which will impact consuming tool(s).

• [x] existing integration tests pass

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[sabbour]

Can we bump the version and add a breaking change comment to the release notes when cutting the release?

[davidgamero]

Can we bump the version and add a breaking change comment to the release notes when cutting the release?

yes for sure! atm the flow is to have a dedicated release PR with the changelog entry, so I'll put it there