Use local cache for yarn commands

[corradio]

No description provided.

[madsnedergaard]

Additional info from @corradio:

The cache location varies depending on the node image used (some platforms put the yarn cache different places), but maybe we could maintain a mapping between the image name and cache location.

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I noticed brick was a bit slow during the prepare stage when using yarn.
This is due to the fact that any change to the package.json or yarn.lock will cause yarn to run from scratch (it has to re-download all packages).
Turns out the yarn cache can be cached using this mount command:

It will make sure to avoid re-downloading packages that are already in the cache. It speeds up quite a lot the build in cases where one adds just a single package!