Enable poetry cache in CI

[andreacamisa]

When docker layers are not found in the cache, poetry install gets executed again, downloading all packages each time. Making sure that poetry looks up packages in the cache would speed up re-building of those layers.

[andreacamisa]

Tried to reproduce approach explained here but cache mounts with "from" argument are not actually bound to the specified directories. (Also, cache name with hash of poetry lock was not computed correctly)

Will need to wait until the cache mounts are implemented: moby/buildkit#1512
Workaround (but it's not worth it right now): docker/buildx#244 (comment)

Further references:

• Cache action: https://github.com/actions/cache
  • Examples https://github.com/actions/cache/blob/main/examples.md#python---pip
• Docker cache mounts: https://docs.docker.com/engine/reference/builder/#run---mounttypecache
  (cache mounts are BuildKit managed and thus cannot control their location. Cannot use bind mounts because read only)
• Docker named contexts: https://docs.docker.com/build/ci/github-actions/named-contexts/