Explain how to use custom setup cmd

doc/ci/examples/license_management.md

@@ -7,8 +7,7 @@ is needed.
 This example shows how to run the License Management tool on your
 project's dependencies by using GitLab CI/CD.
 
-First, you need GitLab Runner with [docker-in-docker executor](../docker/using_docker_build.md#use-docker-in-docker-executor).
-You can then add a new job to `.gitlab-ci.yml`, called `license_management`:
+You can add a new job to `.gitlab-ci.yml`, called `license_management`:
 
 ```yaml
 license_management:
@@ -23,10 +22,41 @@ license_management:
     paths: [gl-license-management-report.json]
 ```
 
-The above example will create a `license_management` job in the `test` stage and will create the required report artifact. Check the
-[Auto-DevOps template](https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/Auto-DevOps.gitlab-ci.yml)
+The above example will create a `license_management` job in the `test` stage
+and will create the required report artifact. Check the [Auto-DevOps
+template](https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/Auto-DevOps.gitlab-ci.yml)
 for a full reference.
 
+## Install custom project dependencies
+
+The `license_management` image already embeds many auto-detection scripts, languages, 
+and packages. Nevertheless, it's almost impossible to cover all cases, for all projects.
+That's why sometimes it's necessary to install extra packages, or to have extra steps
+in the project automated setup, like the download and installation of a certificate. 
+For that, a `SETUP_CMD` environment variable can be passed to the container,
+with the required commands to run before license detection.
+
+If present, this variable will override the setup step necessary to install all the packages
+of your application (ex: for a project with a `Gemfile`, the setup step will be `bundle install`).
+
+Example:
+
+```yaml
+license_management:
+  image: 
+    name: "registry.gitlab.com/gitlab-org/security-products/license-management:$CI_SERVER_VERSION_MAJOR-$CI_SERVER_VERSION_MINOR-stable"
+    entrypoint: [""]
+  stage: test
+  variables:
+    SETUP_CMD=./my-custom-install-script.sh
+  allow_failure: true
+  script:
+    - /run.sh analyze .
+  artifacts:
+    paths: [gl-license-management-report.json]
+```
+
+In this example, `my-custom-install-script.sh` is a shell script at the root of the project.
 
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