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ARROW-7136: [Rust] Added caching to the docker image

This allow us to build the crate without having to re-build nor download all its dependencies at the expense of a larger (build) image.

Here I am extending the ideas of a non-workspace rust project to a workspace rust project. See [here](https://github.com/rust-lang/cargo/issues/2644) for a _long_ discussion on the issue. Essentially, it is not easy to do this atm with cargo.

IMO I think that this is worth it, but I need your help to judge it; it incurs some maintenance (duplication of configurations).

Closes #7799 from jorgecarleitao/docker_caching

Lead-authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Co-authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>