Improve image caching strategy for PR tests

[spwoodcock]

Describe the bug

• Caching improves the speed and efficiency of github workflows - we currently cache quite aggressively.
• However, if there is for example a database migration in a PR, but the cached CI image does not include the migration, then the PR test backend test will fail.
• Someone must delete the github action cache so the CI image is rebuilt.

Expected behavior

• Consider a better caching strategy so no manual intervention is necessary.

[spwoodcock]

https://www.blacksmith.sh/blog/cache-is-king-a-guide-for-docker-layer-caching-in-github-actions

https://evilmartians.com/chronicles/build-images-on-github-actions-with-docker-layer-caching#the-cache-dance-off