build: use Minimus Rust builder

[emiioan]

Replace the public rust:latest builder with reg.mini.dev/rust:latest and run the build stage as root for package installation.

Validated:

• docker build --progress=plain --target runtime -t cachey:minimus-rust-test .
• docker run --rm cachey:minimus-rust-test --help
• cargo +nightly fmt -- --check
• cargo nextest run --all-features (124 passed)
• cargo clippy --all-features --all-targets -- -D warnings --allow deprecated (current-main Rust 1.97 reports 15 useless-borrows-in-formatting warnings in unchanged tests; all other lints pass when that lint is allowed)
• cargo deny check with cargo-deny 0.19.7
• cargo metadata --locked --format-version 1

[greptile-apps]

Greptile Summary

Replaces the public Rust builder image with the Minimus Rust image and explicitly selects the root user for the build stage.

Confidence Score: 5/5

The PR appears safe to merge with no concrete changed-code failures identified.

The builder-stage user does not propagate into the independently declared runtime stages, and the supplied validation confirms that the new builder produces an executable runtime image.

Important Files Changed

Filename	Overview
Dockerfile	Updates only the builder-stage image and user; the root selection remains isolated from the separately based debug and runtime stages.

_{Reviews (1): Last reviewed commit: "build: use Minimus Rust builder" | Re-trigger Greptile}