Improve container image build performance

[rokuosan]

Summary

• reduce pull request build time by building only linux/amd64 in CI while keeping multi-arch builds for pushes and tags
• speed up Docker builds with BuildKit cache mounts and a smaller build context
• slim down the runtime image by building a static binary and running it as a non-root user
• ignore local .cache artifacts generated during development

Why

The container image workflow was spending unnecessary time on emulated multi-platform builds in pull requests.
This change keeps release builds intact while making PR validation faster and tightening the runtime image setup.

[Copilot]

Pull request overview

This PR optimizes container build and CI performance by reducing unnecessary multi-arch work in pull requests, improving Docker build caching, tightening the build context, and hardening the runtime image.

Changes:

• Build only linux/amd64 for PRs while keeping multi-arch builds for pushes/tags.
• Speed up Docker builds via BuildKit cache mounts and a smaller build context (.dockerignore).
• Produce a slimmer runtime image by building a static binary and running as a non-root user.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docker/Dockerfile	Adds BuildKit cache mounts, builds a static binary, and switches runtime to non-root.
.github/workflows/build-container-image.yaml	Skips QEMU on PRs and conditionally limits PR builds to linux/amd64; adjusts cache/provenance settings.
.dockerignore	Reduces Docker build context size by excluding VCS/editor/dev artifacts and built docs output.
.gitignore	Ignores local .cache artifacts.

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