Perform some optimisations on the CI Docker image (#2138)

* Perform some optimisations on the CI Docker image:
- Break down the `install_rust` stage into smaller stages that can advance concurrently;
- Invoke `yum updateinfo` once, in bare_base_image;
- Use the unstable sparse registry feature to speed up `cargo`'s registry operations.

* Fix ARG variable.

* Enable buildkit.