ci: 🎡 use cache (gha) when building the docker images

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Hopefully it will fit in the 10GB.

See
https://github.com/docker/build-push-action/blob/master/docs/advanced/cache.md#cache-backend-api
and https://github.com/moby/buildkit/tree/master#github-actions-cache-experimental

[severo]

Results

First attempt: https://github.com/huggingface/datasets-server/runs/6595660098?check_suite_focus=true

• admin: built in 2m 42s.
• api: built in 2m 39s.
• worker: built in 14m 58s.

Second attempt (relaunching the build manually): https://github.com/huggingface/datasets-server/runs/6595942870?check_suite_focus=true

• admin: built in 3m 10s. <- NO CACHE USED ?!
• api: built in 2m 37s. <- NO CACHE USED ?!
• worker: built in 26s <- CACHE WORKED !!!

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The issue seems to be that only one cache is used for the three workflows in the matrix. Let's find how to get one cache per workflow.

[severo]

YEAHHHHHHHHHHHHH

14 minutes to 43 seconds !!!!!!!!