Speed-up Docker and packages building

[AlekSi]

Findings:

• https://docs.docker.com/build/cache/backends/
• https://docs.docker.com/engine/reference/commandline/buildx_build/
• Issue with sharing cache across jobs docker/buildx#156
• Allow exporting cache mounts for use in CI systems. docker/buildx#244
• https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md
• export and import --mount=type=cache caches moby/buildkit#1474
• Allow controlling cache mounts storage location moby/buildkit#1512
• Deadlock exporting data moby/buildkit#2950
• https://github.com/moby/buildkit#export-cache
• build time only -v / --volume option moby/moby#14080
• https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/

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• BuildKit cache (--cache-from / --cache-to) is almost useless in our case
• Re-using RUN --mount cache is a life saver
• Self-hosted buildx instance with GC disabled is great
• arm/v7 and arm/v6 compilation is very slow even with Go build cache, we might want to use Go's cross-compilation after all