Add support for railpack build cache

What problem will this feature address?

Railpack build is able to use a cache so that future builds are faster.

There are many ways to enable cache for buildkit (which railpack uses under the hood): https://github.com/moby/buildkit#inline-push-image-and-cache-together

• Inline: this just puts the cache inside the generated image
• Registry: pushes a cache container to a registry (can be used with dokploy registries)
• Local directory: puts the cache into a certain directory
• Github actions: not applicable

Describe the solution you'd like

The best point would be to use Registry cache, appending these lines to the build command here: https://github.com/Dokploy/dokploy/blob/1c6fdc1b4379bf6c17611ffc2c8065526f80f20b/packages/server/src/utils/builders/railpack.ts#L48

  --export-cache type=registry,ref=localhost:5000/myrepo:buildcache \
  --import-cache type=registry,ref=localhost:5000/myrepo:buildcache

Describe alternatives you've considered

N/A

Additional context

I'm willing to work on a PR for this, the UI changes would be these:

• When Railpack is selected as the builder, show a "Enable Cache" option, which requires a registry to be set up. When it's enabled, add the relevant --import-cache and --export-cache commands to the railpack build command

Will you send a PR to implement it?

Yes

Start with packages/server/src/utils/builders/railpack.ts at the linked build-command entry point, then trace the Railpack builder settings in the UI and registry configuration. Confirm how builder selection and registry requirements are represented before deciding where the cache option belongs. Done means the option is available only for Railpack, requires a configured registry, and the generated build command includes the requested cache settings when enabled.