feat(dockerfile-llb-frontend): Add the initial LLB frontend code.

This code results in a working LLB plugin that can then be consumed with
docker to build a unikraft app.

Includes unit tests for the core build part, end-to-end tests suites for
BuildKit and Docker, and a comprehensive readme.

The end to end tests build the app-helloworld.

The BuildKit test suite is skipped by default as the docker one
does the same test of correctness. Its usefulness is mostly for local
debugging.

Add a GitHub Action to build and push the kraftkit.sh/llb image.

GitHub-Fixes: #707
GitHub-Fixes: #708

Signed-off-by: Jakub Ciolek <jakub@ciolek.dev>

.github/workflows/dockerfile-llb-frontend.yaml

@@ -0,0 +1,36 @@
+name: tools/dockerfile-llb-frontend
+
+on:
+  push:
+    branches: [stable]
+    paths:
+      - 'tools/dockerfile-llb-frontend/**'
+      - '.github/workflows/dockerfile-llb-frontend.yaml'
+
+jobs:
+  plugin-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Login to Registry
+        uses: docker/login-action@v2
+        with:
+          registry: index.unikraft.io
+          username: ${{ secrets.REG_USERNAME }}
+          password: ${{ secrets.REG_TOKEN }}
+
+      - name: Build OCI image for the LLB plugin
+        uses: docker/build-push-action@v4
+        with:
+          context: ./tools/dockerfile-llb-frontend
+          file: ./tools/dockerfile-llb-frontend/Dockerfile
+          tags: index.unikraft.io/kraftkit.sh/llb:latest
+          push: true
+          platforms: linux/amd64
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

tools/dockerfile-llb-frontend/.dockerignore

@@ -0,0 +1,12 @@
+*/.vscode*
+test/
+*_test.go
+*.bak
+*.swp
+*.idea
+*.orig
+.DS_Store
+*.md
+.git/
+.gitignore
+Dockerfile

tools/dockerfile-llb-frontend/Dockerfile

@@ -0,0 +1,9 @@
+FROM golang:1.20 AS builder
+
+WORKDIR /src
+COPY . ./
+RUN CGO_ENABLED=0 go build -o /kraft-llb-plugin --ldflags "-s -w"
+
+FROM scratch
+COPY --from=builder /kraft-llb-plugin /bin/kraft-llb-plugin
+ENTRYPOINT ["/bin/kraft-llb-plugin"]

tools/dockerfile-llb-frontend/README.md

@@ -0,0 +1,97 @@
+# KraftKit LLB Plugin
+
+This KraftKit LLB Plugin enables running Unikraft builds using Docker commands.
+
+It eliminates the need for installing KraftKit or Unikraft-specific dependencies on your machine by executing builds in containers.
+
+The result is a Unikraft image saved in your local Docker registry.
+
+**Please note:** If you are unfamiliar with Unikraft, KraftKit, or unikernels, kindly refer to the [Unikraft Documentation](https://unikraft.org/docs/getting-started/).
+
+## Prerequisites
+
+- Docker with BuildKit enabled (this is the default setting from Docker v23.0 onwards)
+- Go v1.20 or later
+
+## Usage
+
+There are two ways to use the LLB plugin:
+
+### 1. Docker-based Usage
+
+Build the Docker image containing the plugin:
+
+```sh
+docker build . -t kraftkit.sh/llb
+```
+
+Modify your Kraftfile file by adding the following line at the top:
+
+```yaml
+#syntax=kraftkit.sh/llb:latest
+```
+
+Now, run the Docker build:
+
+```sh
+docker build -f test/apps/app-helloworld/kraft.yaml test/apps/app-helloworld
+```
+
+See [Docker docs about dynamic frontends](https://docs.docker.com/build/dockerfile/frontend/) for more information.
+
+### 2. Direct Binary Usage (Debug Mode)
+
+In this mode, you can output the LLB graph for inspection with buildkit.
+
+First, build the project:
+
+```sh
+go build .
+```
+
+Then, run the built binary from within a Unikraft app directory:
+
+```sh
+dockerfile-llb-frontend --llb-stdout=true | buildctl debug dump-llb
+```
+
+To learn more about this, see this [BuildKit doc](https://github.com/moby/buildkit/blob/master/examples/README.md).
+
+## Contributing
+
+We warmly welcome contributions in the form of tests, bug reports, and feature requests.
+
+For discussions or queries, join us on Discord: https://bit.ly/UnikraftDiscord
+
+### Testing
+
+The project has unit tests for the core build part (./build/build_test.go) and end-to-end
+tests suites in the test/apps directory, one for BuildKit and one for Docker.
+
+The Docker suite calls the docker-backing BuildKit daemon so in a way the BuildKit tests
+are redundant, thus we omit them from the CI flow. They come in handy if you want to
+run a BuildKit daemon with a debugger.
+
+To run these tests:
+
+Docker:
+
+```sh
+go test ./test/docker -v
+```
+
+BuildKit (you have to run buildkit daemon):
+
+To spawn a BuildKit daemon, run:
+
+```sh
+buildkitd
+```
+
+Then run the test suite:
+
+```sh
+go test ./test/buildkit -v
+```
+
+The -v option gives you the output of the BuildKit or Docker runs.