Store the docker images in a build cache

[llunaCreixent]

If we use the docker images in Docker Hub, we could reduce further the build time by using the build cache in the GH Actions, not having to re-download all of the images every time we run the tests workflow.

Some references about best practices for using Docker Hub for CI/CD:

• https://www.docker.com/blog/best-practices-for-using-docker-hub-for-ci-cd/
• https://docs.docker.com/ci-cd/github-actions/