- do not use a intermediate docker image between 'build' and 'test'
- in 'build_deps', avoid downloading the cached image
- in 'prepare', use `opam bundle` to:
- embedded the sources of dependency
- reduce the embedded opam repository to the needed package
Other changes:
- reintroduced 'sudo' in the base image, it is needed by `opam depext`
and the job 'publish:doc'
- moved the docker script used by the CI in `scripts/ci/`
