feat: support overriding test image

[vincent-onebrief]

Description

The test resource is currently hard-coded to a specific alpine image. This image is old and contains vulnerabilities. Users should be able to override this image when required.

I did not change the image as that may be considered a breaking change

Related Issue

Related: #700

Checklist before merging

• [x] Test, docs, adr added or updated as needed
• [x] Contributor Guide Steps followed