better UX for test infrastructure

[doodlesbykumbi]

Description

I noticed as part of #1249 that in the integration tests we either have secretless and secretless-dev built as part of ./start and the docker-compose.yml. The former is horrible because you potentially rebuild something you already have, which increases build time.

It would be good to

1. Standardise on using pre-built images.
2. If the image doesn't exist we can have a common script that runs before all the ./start scripts and either automatically builds secretless and secretless-dev, or prompts the user before doing so (opt-in).

AC:
TBC

• List of conditions that must be met for this card to be complete (aka Acceptance Criteria)