Improve ADDITIONAL_CA_CERT_BUNDLE docs for CS

doc/user/application_security/container_scanning/index.md

@@ -177,7 +177,7 @@ scanning by using the following environment variables:
 
 | Environment Variable           | Default       | Description |
 | ------------------------------ | ------------- | ----------- |
-| `ADDITIONAL_CA_CERT_BUNDLE`    | `""`          | Bundle of CA certs that you want to trust. |
+| `ADDITIONAL_CA_CERT_BUNDLE`    | `""`          | Bundle of CA certs that you want to trust. See [Using a custom SSL CA certificate authority](#using-a-custom-ssl-ca-certificate-authority) for more details. |
 | `CLAIR_DB_CONNECTION_STRING`   | `postgresql://postgres:password@clair-vulnerabilities-db:5432/postgres?sslmode=disable&statement_timeout=60000` | This variable represents the [connection string](https://www.postgresql.org/docs/9.3/libpq-connect.html#AEN39692) to the [PostgreSQL server hosting the vulnerabilities definitions](https://hub.docker.com/r/arminc/clair-db) database and **shouldn't be changed** unless you're running the image locally as described in the [Running the standalone container scanning tool](#running-the-standalone-container-scanning-tool) section. The host value for the connection string must match the [alias](https://gitlab.com/gitlab-org/gitlab/-/blob/898c5da43504eba87b749625da50098d345b60d6/lib/gitlab/ci/templates/Security/Container-Scanning.gitlab-ci.yml#L23) value of the `Container-Scanning.gitlab-ci.yml` template file, which defaults to `clair-vulnerabilities-db`. |
 | `CLAIR_DB_IMAGE`               | `arminc/clair-db:latest` | The Docker image name and tag for the [PostgreSQL server hosting the vulnerabilities definitions](https://hub.docker.com/r/arminc/clair-db). It can be useful to override this value with a specific version, for example, to provide a consistent set of vulnerabilities for integration testing purposes, or to refer to a locally hosted vulnerabilities database for an on-premise offline installation. |
 | `CLAIR_DB_IMAGE_TAG`           | `latest`      | (**DEPRECATED - use `CLAIR_DB_IMAGE` instead**) The Docker image tag for the [PostgreSQL server hosting the vulnerabilities definitions](https://hub.docker.com/r/arminc/clair-db). It can be useful to override this value with a specific version, for example, to provide a consistent set of vulnerabilities for integration testing purposes. |
@@ -217,6 +217,23 @@ GitLab 13.0 and later doesn't support [`only` and `except`](../../../ci/yaml/REA
 When overriding the template, you must use [`rules`](../../../ci/yaml/README.md#rules)
 instead.
 
+## Using a custom SSL CA certificate authority
+
+You can use the `ADDITIONAL_CA_CERT_BUNDLE` environment variable to configure a custom SSL CA certificate authority, which is used to verify the peer when fetching Docker images from a registry which uses HTTPS. The `ADDITIONAL_CA_CERT_BUNDLE` value should contain the [text representation of the X.509 PEM public-key certificate](https://tools.ietf.org/html/rfc7468#section-5.1). For example, to configure this value in the `.gitlab-ci.yml` file, use the following:
+
+```yaml
+container_scanning:
+  variables:
+    ADDITIONAL_CA_CERT_BUNDLE: |
+        -----BEGIN CERTIFICATE-----
+        MIIGqTCCBJGgAwIBAgIQI7AVxxVwg2kch4d56XNdDjANBgkqhkiG9w0BAQsFADCB
+        ...
+        jWgmPqF3vUbZE0EyScetPJquRFRKIesyJuBFMAs=
+        -----END CERTIFICATE-----
+```
+
+The `ADDITIONAL_CA_CERT_BUNDLE` value can also be configured as a [custom variable in the UI](../../../ci/variables/README.md#create-a-custom-variable-in-the-ui), either as a `file`, which requires the path to the certificate, or as a variable, which requires the text representation of the certificate.
+
 ### Vulnerability allowlisting
 
 To allowlist specific vulnerabilities, follow these steps: