Avoid API limit by querying Docker Hub unauthenticated in Github Actions, and build only if needed

[Quetzacoalt91]

Questions	Answers
Description?	Building all the PrestaShop images fails because of the number of API calls allowed reached in a limited period of time. Pulls from Docker Hub could be allowed without limit when we are not authenticated on GitHub Actions environments. This PR limits the risk of quota exceeded by: pulling without being authenticated, building images only if a change has been found.
Type?	improvement
BC breaks?	Nope
Deprecations?	Nope
Fixed ticket?	/
Sponsor company	@PrestaShopCorp
How to test?	Builds succeed on a fork Job without any change -> No image is built (https://github.com/Quetzacoalt91/docker/actions/runs/17497546687) Job from a commit with a diff in the folder images/1.7.4.0 -> Build of 1.7.4.0 (https://github.com/Quetzacoalt91/docker/actions/runs/17497546687/job/49702193446)