fix: replace --mount=type=cache with dependency layer caching

[mikkeldamsgaard]

Summary

• Replaced --mount=type=cache with the "empty main" dependency layer caching trick in both Dockerfile and jyq.Dockerfile
• --mount=type=cache does not persist across GitHub Actions runners, causing full Rust recompilation on every release build
• The dependency layer caching pattern (copy Cargo.toml/lock → build with dummy main.rs → remove initium artifacts → copy real source → rebuild) leverages Docker's built-in layer cache and works reliably with GitHub Actions' type=gha cache

Test plan

• Verify Docker build succeeds locally: docker build -t initium .
• Verify jyq variant builds: docker build -f jyq.Dockerfile -t initium-jyq .
• Confirm CI passes
• Verify next release build is faster (dependency layer cached)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the project’s container build process to use Docker layer caching (via the “empty main.rs” dependency-build pattern) instead of BuildKit --mount=type=cache, improving cache reuse across GitHub Actions runners. It also documents the change in the changelog.

Changes:

• Replaced --mount=type=cache usage with a dependency-layer caching approach in both Docker build variants.
• Adjusted final-stage binary copy paths to pull directly from target/release/initium.
• Added a changelog entry under [Unreleased] describing the Docker caching fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Dockerfile	Switches to dependency-layer caching to speed up repeat builds in CI and updates final binary copy path.
jyq.Dockerfile	Applies the same caching strategy for the jq/yq Alpine variant and updates final binary copy path.
CHANGELOG.md	Documents the Docker build caching change under the Unreleased “Fixed” section.

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