feat: implement a simple docker file to run cli commands (#14111)

This PR implements a minimalistic Dockerfile which runs the required CLI
commands for Librarian.

Testing steps:

```
docker build . -t <image-name>
```

Command:
```
docker run --rm <image-name>
```

Output:
```
'generate' command executed.
```

Fixes: googleapis/librarian#899

Author: ohmayr

.generator/Dockerfile

@@ -0,0 +1,71 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+FROM marketplace.gcr.io/google/ubuntu2404 AS builder
+
+# TODO(https://github.com/googleapis/librarian/issues/901): Install the necssary dependencies and build tools.
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    # Essential for compiling C code
+    build-essential \
+    # For downloading secure files
+    wget \
+    ca-certificates \
+    # --- Critical libraries for a complete Python build ---
+    libssl-dev \
+    zlib1g-dev \
+    libbz2-dev \
+    libffi-dev \
+    libsqlite3-dev \
+    libreadline-dev \
+    # ------------------------------------------------------
+    && apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Set up environment variables for tool versions to make updates easier.
+# TODO(): Downgrade to Python `3.11` if `3.13.5` is incompatible with googleapis. 
+ENV PYTHON_VERSION=3.13.5
+
+# Create a symbolic link for `python3` to point to our specific version.
+ENV PATH /usr/local/bin/python3.13:$PATH
+
+# Install Python from source
+RUN wget https://www.python.org/ftp/python/${PYTHON_VERSION}/Python-${PYTHON_VERSION}.tgz && \
+    tar -xvf Python-${PYTHON_VERSION}.tgz && \
+    cd Python-${PYTHON_VERSION} && \
+    ./configure --enable-optimizations && \
+    make altinstall && \
+    cd / && \
+    rm -rf Python-${PYTHON_VERSION}*
+
+# TODO(https://github.com/googleapis/librarian/issues/904): Install protoc for gencode.
+
+# TODO(https://github.com/googleapis/librarian/issues/903): Install Bazelisk for building a client library.
+
+# TODO(https://github.com/googleapis/librarian/issues/902): Create a dedicate non-root user and 
+# switch to the non-root user to run subsequent commands.
+
+# Set the working directory in the container.
+WORKDIR /app
+
+# TODO(https://github.com/googleapis/librarian/issues/907): Install Python dependencies from requirements.in.
+# TODO(https://github.com/googleapis/librarian/issues/905): Install Synthtool by cloning its repo.
+# TODO(https://github.com/googleapis/librarian/issues/906): Clone googleapis and run bazelisk build.
+
+# Copy the CLI script into the container and set ownership.
+# No other files or dependencies are needed for this simple script.
+COPY cli.py .
+
+# Set the entrypoint for the container to run the script.
+ENTRYPOINT ["python3.13", "./cli.py"]

.generator/cli.py

@@ -0,0 +1,50 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import argparse
+import sys
+
+def handle_configure(dry_run=False):
+    # TODO(https://github.com/googleapis/librarian/issues/466): Implement configure command.
+    print("'configure' command executed.")
+
+def handle_generate(dry_run=False):
+    # TODO(https://github.com/googleapis/librarian/issues/448): Implement generate command.
+    print("'generate' command executed.")
+
+def handle_build(dry_run=False):
+    # TODO(https://github.com/googleapis/librarian/issues/450): Implement build command.
+    print("'build' command executed.")
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description="A simple CLI tool.")
+    subparsers = parser.add_subparsers(dest="command", required=True, help="Available commands")
+
+    # Define commands
+    for command_name, help_text in [
+        ("configure", "Onboard a new library or an api path to Librarian workflow."),
+        ("generate", "generate a python client for an API."),
+        ("build", "Run unit tests via nox for the generated library.")
+    ]:
+        handler_map = {"configure": handle_configure, "generate": handle_generate, "build": handle_build}
+        parser_cmd = subparsers.add_parser(command_name, help=help_text)
+        parser_cmd.set_defaults(func=handler_map[command_name])
+
+    if len(sys.argv) == 1:
+        parser.print_help(sys.stderr)
+        sys.exit(1)
+
+    args = parser.parse_args()
+    args.func(args)

.generator/test_cli.py

@@ -0,0 +1,28 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from cli import handle_generate, handle_build, handle_configure
+
+
+def test_handle_configure_dry_run():
+    # This is a simple test to ensure that the dry run command succeeds.
+    handle_configure(dry_run=True)
+
+def test_handle_generate_dry_run():
+    # This is a simple test to ensure that the dry run command succeeds.
+    handle_generate(dry_run=True)
+
+def test_handle_build_dry_run():
+    # This is a simple test to ensure that the dry run command succeeds.
+    handle_build(dry_run=True)

.github/workflows/generator.yml

@@ -0,0 +1,30 @@
+on:
+  pull_request:
+    branches:
+      - main
+name: generator
+
+permissions:
+  contents: read
+
+jobs:
+  test_generator_cli:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      # Use a fetch-depth of 2
+      # See https://github.com/googleapis/google-cloud-python/issues/12013
+      # and https://github.com/actions/checkout#checkout-head.
+      with:
+        fetch-depth: 2
+    - name: Setup Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3.10"
+    - name: Install pytest
+      run: |
+        python -m pip install pytest
+    - name: Run generator_cli tests
+      run: |
+        pytest .generator/test_cli.py