docker: improve Rust dependency caching in CI to avoid rebuilding

[gustavovalverde]

Motivation

When files are copied into the Dockerfile with the COPY command, the cache can get invalidated. We could improve this by using the --link option which creates more independent layers to avoid rebuilding when not needed

Specifications

Documentation: https://docs.docker.com/engine/reference/builder/#copy---link

[gustavovalverde]

Extra documentation:

• https://hackmd.io/@kobzol/S17NS71bh
• Support mirror/recursive COPY moby/moby#39530
• cargo build --dependencies-only rust-lang/cargo#2644
• (Option to) Fingerprint by file contents instead of mtime rust-lang/cargo#6529
• Cargo mtime issue preventing effective caching on git dependencies Swatinem/rust-cache#155

Partial solutions:

• https://ectobit.com/blog/rust-container-image-buildkit-buildx/
• paradigmxyz/reth@a22c274
• Allow controlling cache mounts storage location moby/buildkit#1512 (comment)

[gustavovalverde]

This has become a cost issue, so we should prioritize it.