feat: example for build matrixing with RCL

.github/workflows/100-build-rcl.yml

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+name: bluebuild
+on:
+  push:
+    paths:
+      - "100-rcl/**"
+      - ".github/workflows/100-build-rcl.yml"
+    branches:
+      - main
+
+  pull_request:
+  workflow_dispatch: # allow manually triggering builds
+jobs:
+  rcl:
+    name: Build recipe files with rcl
+    runs-on: ubuntu-latest
+    outputs:
+      recipe-matrix: ${{ steps.rcl.outputs.recipe-matrix }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install rcl
+        run: |
+          git clone https://github.com/ruuda/rcl.git /tmp/rcl
+          cd /tmp/rcl && cargo build --release
+          sudo cp target/release/rcl /usr/local/bin/
+
+      - name: Build recipe files
+        id: rcl
+        run: |
+          RCL_OUTPUT=$(rcl build ./100-rcl/recipes.rcl)
+          RECIPE_FILENAMES=$(echo "$RCL_OUTPUT" | awk '{ print $2 }' | sed -e 's/^recipes\///')
+          RECIPE_FILENAMES_JSON=$(echo "$RECIPE_FILENAMES" | jq --compact-output --raw-input '[inputs]')
+          MATRIX_JSON=$(jq -cn --argjson recipes "${RECIPE_FILENAMES_JSON[@]}" '{ recipes: $recipes }')
+
+          echo "recipe-matrix=$MATRIX_JSON" >> $GITHUB_OUTPUT
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: recipes
+          path: ./100-rcl/recipes/
+  bluebuild:
+    name: Build Custom Image
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+    needs: rcl
+    strategy:
+      fail-fast: false # stop GH from cancelling all matrix builds if one fails
+      matrix: ${{ fromJson(needs.rcl.outputs.recipe-matrix) }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/download-artifact@v4
+        with:
+          name: recipes
+          path: ./100-rcl/recipes/
+
+      - name: Build Custom Image
+        uses: blue-build/[email protected]
+        with:
+          recipe: ${{ matrix.recipes }}
+          working_directory: ./100-rcl
+          cosign_private_key: ${{ secrets.SIGNING_SECRET }}
+          registry_token: ${{ github.token }}
+          pr_event_number: ${{ github.event.number }}
+          skip_checkout: true

100-rcl/.gitignore

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+*.yml

100-rcl/README.md

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+# Easy build matrixing with [RCL](https://rcl-lang.org/)
+
+[RCL](https://rcl-lang.org/) is a next-gen configuration language built in Rust which in this context can be used for easy matrixed recipe generation and image building through its simple way of generating configuration files. RCL is a superset of JSON and functions similarly to Jsonnet and Nix, but has a much better syntax and user experience.
+
+The benefits of using a configuration language with built-in matrixing and templating is that you can easily generate multiple similar recipes using ex. different base images and easily introduce differences between them using simple `if` statements and template strings. You could get to the same result by simply using multiple recipe files, one for each image, but when building a lot of very similar images that approach might be cumbersome and require a ton of repetitive work.
+
+This example showcases how to use RCL to generate a bunch recipes and build them all concurrently using with the GitHub Actions build matrix. The build matrix is automatically generated from the output of the `rcl build` command, and the logic of that generation might have to be adjusted to future RCL updates.
+
+- See [recipes.rcl](./recipes.rcl) for the RCL configuration.
+- See [build.yml](./build.yml) for the GitHub Actions workflow.
+
+When working with a third-party configuration language, it is recommended to use the importing feature of the language instead of `from-file`, such that you can leave the `recipes/` directory completely empty or even uncreated. If you still want to use `from-file`, remember that the paths are not relative to the `.rcl` file, but to the `recipes/` directory.   
+See RCL's [documentation for file imports](https://docs.ruuda.nl/rcl/imports/).

100-rcl/cosign.pub

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+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEliM5WSdh8GEB5c9ffB+/RJJAbAkT
+Rkdfr9RsoSjPITV0foGyvoiQ06DCiaTgaj6kBG2rUHPXEgIH5B2IlPJMRw==
+-----END PUBLIC KEY-----

100-rcl/files/scripts/nvidia-setup.sh

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+#!/usr/bin/env bash
+set -oue pipefail
+
+echo "Something nvidia related I guess"

100-rcl/recipes.rcl

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+let project = {
+    name = "test",
+    description = "My test project.",
+    base-images = "ghcr.io/ublue-os"
+};
+
+let gts-version = 40;
+
+{
+    for base-image in ["kinoite", "silverblue"]:
+    for nvidia in [true, false]:
+    for image-version in [gts-version, "latest"]:
+
+    let nvidia-suffix = if nvidia: "-nvidia" else: "-main";
+
+    f"recipes/recipe-{base-image}{nvidia-suffix}-{image-version}.yml": {
+        format = "json",
+        contents = {
+            name = f"{project.name}-{base-image}{nvidia-suffix}",
+            description =
+                f"{project.description} ({base-image}{nvidia-suffix} edition, {if image-version == gts-version: "GTS" else "latest"} version)",
+
+            base-image = f"{project.base-images}/{base-image}{nvidia-suffix}",
+            image-version = image-version,
+
+            if image-version == gts-version:
+                alt-tags = [
+                    "gts"
+                ],
+
+            modules = [
+                {
+                    type = "files",
+                    files = [
+                        {
+                            source = "system",
+                            destination = "/",
+                        },
+                        if nvidia:
+                        {
+                            source = "system_nvidia",
+                            destination = "/",
+                        }
+                    ]
+                },
+                {
+                    type = "default-flatpaks",
+                    user = {
+                        install = [
+                            "org.kde.krita",
+                            "org.fedoraproject.MediaWriter",
+                            if base-image == "kinoite":
+                                "org.kde.elisa",
+                            if base-image == "silverblue":
+                                "com.github.neithern.g4music",
+                        ]
+                    }
+                },
+                if nvidia: {
+                    type = "script",
+                    scripts = ["nvidia-setup.sh"]
+                },
+                { type = "signing" }
+            ]
+        }
+    }
+}