doc: introduce PIP_EXTRA_INDEX_URL

Useful in corporate environment with more than one repository to access.

README.md

@@ -25,6 +25,7 @@ The Python template uses some global configuration used throughout all jobs.
 | `PYTHON_PROJECT_DIR` | Python project root directory                                                         | `.`                |
 | `PYTHON_BUILD_SYSTEM`| Python build-system to use to install dependencies, build and package the project (see below) | _none_ (auto-detect) |
 | `PIP_INDEX_URL`      | Python repository url                                                                 | _none_             |
+| `PIP_EXTRA_INDEX_URL` | Exra Python repository url                                                           | _none_             |
 | `PIP_OPTS`           | pip [extra options](https://pip.pypa.io/en/stable/cli/pip/#general-options)     | _none_             |
 | `PYTHON_EXTRA_DEPS`  | Python extra sets of dependencies to install<br/>For [Setuptools](https://setuptools.pypa.io/en/latest/userguide/dependency_management.html?highlight=extras#optional-dependencies) or [Poetry](https://python-poetry.org/docs/pyproject/#extras) only | _none_ |
 | `PYTHON_REQS_FILE`   | Main requirements file _(relative to `$PYTHON_PROJECT_DIR`)_<br/>For [Requirements Files](https://pip.pypa.io/en/stable/user_guide/#requirements-files) build-system only | `requirements.txt` |
@@ -364,3 +365,25 @@ The template handles both classic variable and file variable.
 Simply specify :lock: `$GIT_USERNAME` and :lock: `$GIT_PASSWORD` as secret project variables.
 
 Note that the password should be an access token (preferably a [project](https://docs.gitlab.com/ee/user/project/settings/project_access_tokens.html) or [group](https://docs.gitlab.com/ee/user/group/settings/group_access_tokens.html) access token) with `write_repository` scope and `Maintainer` role.
+
+#### Pip repositories
+
+When depending on Python packages published in [GitLab's packages registry](https://docs.gitlab.com/ee/user/packages/pypi_repository/), it could be useful to configure a group level Package.
+But such repository will require an authenticated access.
+
+To do so, simply set the `PIP_INDEX_URL` and use the CI job token.
+
+```YAML
+variables:
+  PIP_INDEX_URL: "${CI_SERVER_PROTOCOL}://gitlab-ci-token:${CI_JOB_TOKEN}@${CI_SERVER_HOST}:${CI_SERVER_PORT}/api/v4/groups/<group-id>/-/packages/pypi/simple"
+```
+
+In a corporate environment, you can be faced to two repositories: the corporate proxy-cache and the project repository.
+
+Simply use both `PIP_INDEX_URL` and `PIP_EXTRA_INDEX_URL`.
+
+```YAML
+variables:
+  PIP_INDEX_URL: "https://cache.corp/repository/pypi/simple"
+  PIP_EXTRA_INDEX_URL: "${CI_SERVER_PROTOCOL}://gitlab-ci-token:${CI_JOB_TOKEN}@${CI_SERVER_HOST}:${CI_SERVER_PORT}/api/v4/groups/<group-id>/-/packages/pypi/simple"
+```