Update CI schema tests for `image:docker`

The following discussion from !112907 (merged) should be addressed:

• @mgandres started a discussion:

  Suggestion (non-blocking): It would be good to add a link to the relevant docs here. We should also add tests for this in spec/frontend/editor/schema/ci.

  But given the amount of effort already put into this Community contribution, and since schema linting won't break this feature I'm okay with addressing this as a follow-up.

  Works great on the pipeline editor! 🎉

  Valid Config	With Error