Update dependency scanning to add new variable

doc/user/application_security/dependency_scanning/index.md

@@ -141,6 +141,7 @@ using environment variables.
 | `DS_ANALYZER_IMAGES`                    | Comma separated list of custom images. The official default images are still enabled. Read more about [customizing analyzers](analyzers.md). |
 | `DS_ANALYZER_IMAGE_PREFIX`              | Override the name of the Docker registry providing the official default images (proxy). Read more about [customizing analyzers](analyzers.md). |
 | `DS_ANALYZER_IMAGE_TAG`                 | Override the Docker tag of the official default images. Read more about [customizing analyzers](analyzers.md). |
+| `DS_PYTHON_VERSION`                     | Version of Python. If set to 2, dependencies are installed using Python 2.7 instead of Python 3.6. ([Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/12296) in GitLab 12.1)|
 | `DS_DEFAULT_ANALYZERS`                  | Override the names of the official default images. Read more about [customizing analyzers](analyzers.md). |
 | `DS_DISABLE_REMOTE_CHECKS`              | Do not send any data to GitLab. Used in the [Gemnasium analyzer](#remote-checks). |
 | `DS_PULL_ANALYZER_IMAGES`               | Pull the images from the Docker registry (set to `0` to disable). |