cache the cargo registry cache directories to speed up build times

[khumps]

According to the cargo docs the local cargo package registry is safe to cache. Doing so massively speeds up docker recompile times because the docker build does not need to re-fetch any packages if the cargo manifest or lockfile hasn't changed (almost half the build time in local development).

[Nutomic]

Try updating the docker image to 1.70 which fetches the package registry more efficiently. Then this shouldnt make much difference anymore.