Docker nightly builds in GitHub Actions

[jpraynaud]

Why

We want to detect when the Docker image builds are failing. They are not currently tested in the CI because they are too long to run on each PR.

What

Implement scheduled CI runs for Docker builds with the available cron scheduler of GitHub Actions.

How

• Create new scheduled workflow to build all the Docker images that are not built in the CI (not suffixed by .ci)
• Schedule the run every night at 2 am GMT