## Context
Using Railpack locally with BuildKit (`docker buildx build ... --build-arg BUILDKIT_SYNTAX=...` with a local registry): cache images are pushed after the build but never pulled before it, so every build is cold. `frontend.go`'s `Build()` never set `CacheImports` on its internal `client.Solve()` call.

## Verified resolution
Maintainer iloveitaly (building on community fix PR #595 from mvanhorn plus review from mabadir): fixed the frontend import cache and added cache import/export support to the `railpack build` command, included in the next release.