feat: add Docker volume cache mounting for reproducible builds

[Copilot]

Adds opt-in cache mounting to speed up sequential reproducible builds in workspaces and CI/CD by reusing compiled dependencies and downloaded crates across Docker container invocations.

Changes

• CLI flags: Added --mount-target-cache and --mount-cargo-cache to cargo near build reproducible-wasm

  • Both default to false for backward compatibility
  • Also available in cargo near deploy reproducible-wasm via flag flattening

• Volume mounting: Extended additional_docker_args() to construct volume mounts when flags are enabled

  • ./target → /target_cache with CARGO_TARGET_DIR=/target_cache
  • ~/.cargo → /usr/local/cargo (standard CARGO_HOME in Rust images)
  • Handles CARGO_HOME env var and cross-platform home directory detection

• Documentation: Added README section explaining cache flags and reproducibility trade-offs

Usage

# Sequential workspace builds with shared cache
cd omni-bridge
cargo near build reproducible-wasm --mount-target-cache --mount-cargo-cache
cd ../another-contract
cargo near build reproducible-wasm --mount-target-cache --mount-cargo-cache

Cache reuse pattern: EC → 1EC, EC → 2EC, EC → 3EC where EC is external cache, enabling significant build time reduction when building multiple contracts or rebuilding the same contract.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: supply external cache to build reproducible-wasm with cargo-near</issue_title>
<issue_description>relates to #301

this should at first try to pass external ./target dir to the right place into container, and then, if that can be made to work, ~/.cargo folder too

    /Users/runner/.cargo/bin
    /Users/runner/.cargo/.crates2.json
    /Users/runner/.cargo/.crates.toml
    /Users/runner/.cargo/git
    /Users/runner/.cargo/registry/cache
    /Users/runner/.cargo/registry/index
    ./target

NOTE: original discussion of docker volumes somewhere in tg only mentioned rebuilding the same crate after initial
build for reduced build times, the task has had an obvious scope extension, where a sequence of builds of crates in the same workspace have to be passed initial cache, and then all built sequentially, while sharing the same cache.
Original description:
1i -> 1C
(C stands for Cache, 1i is non-cached build of crate 1, 1C is cached build of crate 1 with cache obtained during 1i )
Extended description:
sequential build of multiple crates or workspace: EC -> 1EC , EC -> 2EC, EC -> 3EC ...
(EC stands for External Cache, 1EC is cached build of crate 1 with cache supplied externally)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat: supply external cache to build reproducible-wasm with cargo-near #308

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[Copilot]

Pull request overview

This PR adds optional Docker volume cache mounting to speed up sequential reproducible builds in workspaces and CI/CD environments by reusing compiled dependencies and downloaded crates across container invocations.

Changes:

• Added --mount-target-cache and --mount-cargo-cache CLI flags with default false for backward compatibility
• Extended Docker volume mounting logic to map local ./target and ~/.cargo directories into containers
• Added README documentation explaining cache flags and reproducibility trade-offs

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
cargo-near/src/commands/build/actions/reproducible_wasm/mod.rs	Added mount_target_cache and mount_cargo_cache boolean fields to BuildOpts and propagated them through conversion functions
cargo-near-build/src/types/near/docker_build/mod.rs	Added mount_target_cache and mount_cargo_cache fields to DockerBuildOpts struct
cargo-near-build/src/near/docker_build/mod.rs	Implemented additional_docker_args function to construct volume mounts and environment variables when cache flags are enabled
README.md	Added "Caching for faster builds" section documenting the new flags and reproducibility caveats

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[Copilot]

The cargo home directory is not created before calling dunce::canonicalize, unlike the target directory (lines 141-143). If the directory doesn't exist (e.g., on a fresh system), canonicalize will fail. Add std::fs::create_dir_all(&cargo_home) before the canonicalize call, similar to the target directory handling.