Use the Docker image as a source of cache layers

[macbre]

See https://testdriven.io/blog/faster-ci-builds-with-docker-cache/

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• https://www.docker.com/blog/advanced-dockerfiles-faster-builds-and-smaller-images-using-buildkit-and-multistage-builds/
• https://docs.docker.com/engine/reference/commandline/build/#specifying-external-cache-sources

[macbre]

• Without BuildKit - succeeded in 2m 35s
• With BuildKit - after 665c78f - succeeded in 16s (!)
• With BUILDKIT_INLINE_CACHE=1 - succeeded in 11s (!)