feat: export distribution container build artifacts

Add a new --export-dir flag to the `llama stack build` command that
allows users to export container build artifacts to a specified
directory instead of building the container directly. This feature is
useful for:

- Building containers in different environments
- Sharing build configurations
- Customizing the build process

The exported tarball includes:
- Containerfile (Dockerfile)
- Run configuration file (if building from config)
- External provider files (if specified)
- Build script for assistance

The tarball is named with a timestamp for uniqueness: <distro-name>_<timestamp>.tar.gz

Documentation has been updated in building_distro.md to reflect this new
functionality as well as integration tests.

Signed-off-by: Sébastien Han <seb@redhat.com>

.github/workflows/providers-build.yml

@@ -198,3 +198,55 @@ jobs:
               'source /etc/os-release && echo "$ID"' \
               | grep -qE '^(rhel|ubi)$' \
               || { echo "Base image is not UBI 9!"; exit 1; }
+
+  export-build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Set up Python
+        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
+        with:
+          python-version: '3.10'
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@0c5e2b8115b80b4c7c5ddf6ffdd634974642d182 # v5.4.1
+        with:
+          python-version: "3.10"
+
+      - name: Install LlamaStack
+        run: |
+          uv venv
+          source .venv/bin/activate
+          uv pip install -e .
+
+      - name: Pin template to UBI9 base
+        run: |
+          yq -i '
+            .image_type    = "container" |
+            .image_name    = "ubi9-test" |
+            .distribution_spec.container_image = "registry.access.redhat.com/ubi9:latest"
+          ' llama_stack/templates/starter/build.yaml
+
+      - name: Test the export
+        run: |
+          # Support for USE_COPY_NOT_MOUNT=true LLAMA_STACK_DIR=. will be added in the future
+          uv run llama stack build --config llama_stack/templates/starter/build.yaml --export-dir export
+          for file in export/*; do
+            echo "File: $file"
+            if [[ "$file" == *.tar.gz ]]; then
+              echo "Tarball found"
+              tarball_found=1
+              tar -xzvf "$file" -C export
+            else
+              continue
+            fi
+            break
+          done
+          if [ -z "$tarball_found" ]; then
+            echo "Tarball not found"
+            exit 1
+          fi
+          cd export
+          docker build -t export-test -f ./Containerfile .