[CI] Avoid rebuilding dioxus-cli on every image build

[brendanPro]

Summary: cargo install dioxus-cli --version 0.7.9 dominates the UI Docker stage on cold builds; cache or prebake it.

Parent epic: #44

Objective / Problem

The UI stage installs dioxus-cli from crates.io on every cold build before dx build. That install is large and rarely changes (pinned 0.7.9).

Proposed Solution

Pick one (document choice):

1. Persist CLI via BuildKit cache mount / dedicated layer that only busts when version changes.
2. Or use a thin base image / GHCR cache image that already contains dx 0.7.9.
3. Align with local CONTRIBUTING pin (0.7.9).

QA

• Cold build no longer compiles dioxus-cli from scratch when version unchanged.
• Bumping CLI version intentionally rebuilds/installs the new pin.
• dx build -p proteus-ui still succeeds in the image.