feat(docker): add external build caching

[harryvince]

Changes

• Added optional external BuildKit caching for Dockerfile application builds.
• Added registry cache imports and exports with explicit references and mode=max exports.
• Added preview cache inheritance, overrides, and an option to disable preview caching.
• Added warn-and-retry or strict cache failure handling.
• Added advanced raw cache settings, including persistent per-application local caches under /cache.
• Force rebuilds skip imports but refresh exports; Disable Build Cache disables both.
• Added local cache and Buildx builder cleanup.
• Added API persistence, validation, OpenAPI fields, deployment snapshots, and automated coverage.
• Prepared a companion Dockerfile documentation update.

Issues

• Fixes N/A

Category

• Bug fix
• Improvement
• New feature
• Adding new one click service
• Fixing or updating existing one click service

Preview

AI Assistance

• AI was NOT used to create this PR
• AI was used (please describe below)

If AI was used:

• Tools used: OpenAI Codex
• How extensively: Used for design, implementation, tests, documentation, and review. All changes were reviewed and verified locally by the contributor.

Testing

• 142 focused Pest tests passed with 462 assertions.
• Laravel Pint passed for changed PHP files.
• The Vite production build passed.
• OpenAPI YAML and JSON were regenerated.
• Documentation MDX generation and TypeScript checks passed.

Contributor Agreement

Important

• I have read and understood the contributor guidelines. If I have failed to follow any guideline, I understand that this PR may be closed without review.
• I have searched existing issues and pull requests (including closed ones) to ensure this is not a duplicate.
• I have tested all changes thoroughly with a local development instance and am confident they will work as expected when a maintainer tests them.