docs: add instructions for analyzing GitLab repositories (#361)

Signed-off-by: Nathan Nguyen <nathan.nguyen@oracle.com>

Author: nathanwn

docs/source/pages/using.rst

@@ -11,9 +11,15 @@ Using Macaron
 
 .. _analyze-action:
 
-------------------------------------
+.. contents:: :local:
+
+----------------------------------
+Analyzing a source code repository
+----------------------------------
+
+''''''''''''''''''''''''''''''''''''
 Analyzing a public Github repository
-------------------------------------
+''''''''''''''''''''''''''''''''''''
 
 Macaron can analyze a Github public repository (and potentially the repositories of it dependencies) to determine its SLSA posture following the specification of `SLSA v0.1 <https://slsa.dev/spec/v0.1/>`_.
 
@@ -52,6 +58,38 @@ With the example above, the generated output reports can be seen here:
 - `micronaut-core.html <../_static/examples/micronaut-projects/micronaut-core/analyze_with_repo_path/micronaut-core.html>`__
 - `micronaut-core.json <../_static/examples/micronaut-projects/micronaut-core/analyze_with_repo_path/micronaut-core.json>`__
 
+'''''''''''''''''''''''''''''
+Analyzing a GitLab repository
+'''''''''''''''''''''''''''''
+
+Macaron supports analyzing GitLab repositories, whether they are hosted on `gitlab.com <https://gitlab.com>`_ or on your self-hosted GitLab instance. The set up in these two cases are a little bit different.
+
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+Analyzing a repository on `gitlab.com <https://gitlab.com>`_
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+Analyzing a public repository on `gitlab.com <https://gitlab.com>`_ is quite similar to analyzing a public GitHub repository -- you just need to pass a proper GitLab repository URL to ``macaron analyze``.
+
+To analyze a private repository hosted on ``gitlab.com``, you need to obtain a GitLab access token having at least the ``read_repository`` permission and store it into the ``MCN_GITLAB_TOKEN`` environment variable. For more detailed instructions, see `GitLab documentation <https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html#create-a-personal-access-token>`_.
+
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""
+Analyzing a repository on a self-hosted GitLab instance
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+To analyze a repository on a self-hosted GitLab instance, you need to do the following:
+
+- Add the following ``[git_service.gitlab.self_hosted]`` section into your ``.ini`` config. In the default .ini configuration (generated using ``macaron dump-default`` -- :ref:`see instructions <action_dump_defaults>`), there is already this section commented out. You can start by un-commenting this section and modifying the ``domain`` value with the domain of your self-hosted GitLab instance.
+
+.. code-block:: ini
+
+    # Access to a self-hosted GitLab instance (e.g. your organization's self-hosted GitLab instance).
+    # If this section is enabled, an access token must be provided through the ``MCN_SELF_HOSTED_GITLAB_TOKEN`` environment variable.
+    # The `read_repository` permission is required for this token.
+    [git_service.gitlab.self_hosted]
+    domain = internal.gitlab.org
+
+- Obtain a GitLab access token having at least the ``read_repository`` permission and store it into the ``MCN_SELF_HOSTED_GITLAB_TOKEN`` environment variable. For more detailed instructions, see `GitLab documentation <https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html#create-a-personal-access-token>`_.
+
 -------------------------------------------------
 Verifying provenance expectations in CUE language
 -------------------------------------------------

src/macaron/config/defaults.ini

@@ -74,7 +74,7 @@ domain = github.com
 domain = gitlab.com
 
 # Access to a self-hosted GitLab instance (e.g. your organization's self-hosted GitLab instance).
-# If this section is enabled, an access token must be provided through the `MCN_PUBLICLY_HOSTED_GITLAB_TOKEN` environment variable.
+# If this section is enabled, an access token must be provided through the `MCN_SELF_HOSTED_GITLAB_TOKEN` environment variable.
 # The `read_repository` permission is required for this token.
 # [git_service.gitlab.self_hosted]
 # domain = example.org