fix(builder): Allow custom dockerfile names when building

[craciunoiuc]

Previously, the name was cut from the path, so it used the default 'Dockerfile' name breaking functionality.

Depends on: unikraft-cloud/x#253
Merged

Closes: TOOL-855

TODOs left:
* Bump the library after merging
* Update the golden files for tests when they start behaving

[jedevc]

This shouldn't be merged as is, it should use the kraftfile dockerfile property.

[craciunoiuc]

correct

[Copilot]

Pull request overview

This PR updates the build pipeline to support building rootfs images from Dockerfiles with non-default filenames by introducing an explicit Rootfs.Dockerfile field, wiring it through Kraftfile parsing, and passing the correct BuildKit frontend attributes for Dockerfile selection.

Changes:

• Add FSOpts.Dockerfile and plumb it from Kraftfile (rootfs.source.dockerfile) into builder options.
• Update BuildKit LocalDirs/frontend attrs so the Dockerfile frontend can be pointed at a custom filename.
• Add integration/E2E test coverage for building with a custom Dockerfile name and bump unikraft.com/x/kraftfile.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
internal/builder/rootfs.go	Adjusts Dockerfile build context wiring to BuildKit; updates dockerfile-path validation/error messages.
internal/builder/rootfs_test.go	Adds integration test for custom Dockerfile filename builds.
internal/builder/kraftfile.go	Adapts to new kraftfile FSSource shape and propagates dockerfile to BuildOpts.
internal/builder/kraftfile_test.go	Updates tests for new kraftfile source structure and adds dockerfile-field cases.
internal/builder/build.go	Extends FSOpts with a Dockerfile field.
cmd/unikraft/integration/build_test.go	Adds end-to-end test for rootfs.source.dockerfile in Kraftfile.
go.mod / go.sum	Bumps unikraft.com/x/kraftfile dependency and updates sums.

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