BuildKit cache import/export; private registry auth

CLI

New

• Build: The build command now supports BuildKit cache import and export via --cache-from and --cache-to, using the same syntax as docker buildx. For example: railpack build --cache-from type=registry,ref=my.registry/cache --cache-to type=registry,ref=my.registry/cache,mode=max . by @mvanhorn in https://github.com/railwayapp/railpack/pull/595
• Build: railpack build now uses credentials from your Docker CLI config ($DOCKER_CONFIG, default ~/.docker/config.json) so BuildKit can pull from and push to private registries. Log in with docker login first if needed. by @iloveitaly in https://github.com/railwayapp/railpack/pull/623
• Mise: Updated mise version from v2026.7.2 to v2026.7.5.

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Internal improvements to testing, documentation, and refactoring by @iloveitaly.

Full Changelog: https://github.com/railwayapp/railpack/compare/v0.30.1...v0.31.0