Fix Dockerfile (headroomlabs-ai#2337)

## Description

Optimize the Docker build cache to ensure that Rust/Cargo dependencies
are cached correctly when installing Python packages, thereby speeding
up subsequent image builds.

Closes #

## Type of Change

- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [x] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Dockersfile

## Review Readiness

- [ ] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] I did **not** edit `CHANGELOG.md` — it is generated by
release-please from my Conventional Commit PR title (a CI guard enforces
this)

Co-authored-by: JerrettDavis <mxjerrett@gmail.com>

Author: jerviscui

Dockerfile

@@ -49,7 +49,8 @@ COPY headroom/ headroom/
 
 ARG HEADROOM_EXTRAS=proxy,code
 RUN --mount=type=cache,target=/root/.cache/uv \
-    --mount=type=cache,target=/root/.cargo/registry \
+    --mount=type=cache,target=/usr/local/cargo/registry \
+    --mount=type=cache,target=/usr/local/cargo/git \
     --mount=type=cache,target=/build/target \
     uv pip install --system ".[${HEADROOM_EXTRAS}]"