Merge branch 'docs-cs-default-credentials' into 'master'

Fix default credential information for Container Scanning

See merge request gitlab-org/gitlab!67093

doc/user/application_security/container_scanning/index.md

@@ -160,8 +160,8 @@ You can [configure](#customizing-the-container-scanning-settings) analyzers by u
 | `CS_REGISTRY_INSECURE`         | `"false"`     | Allow access to insecure registries (HTTP only). Should only be set to `true` when testing the image locally. | Trivy. The registry must listen on port `80/tcp`. |
 | `CS_SEVERITY_THRESHOLD`        | `UNKNOWN`     | Severity level threshold. The scanner outputs vulnerabilities with severity level higher than or equal to this threshold. Supported levels are Unknown, Low, Medium, High, and Critical. | Trivy |
 | `DOCKER_IMAGE`                 | `$CI_APPLICATION_REPOSITORY:$CI_APPLICATION_TAG` | The Docker image to be scanned. If set, this variable overrides the `$CI_APPLICATION_REPOSITORY` and `$CI_APPLICATION_TAG` variables. | All |
-| `DOCKER_PASSWORD`              | `$CI_REGISTRY_PASSWORD` | Password for accessing a Docker registry requiring authentication. | All |
-| `DOCKER_USER`                  | `$CI_REGISTRY_USER` | Username for accessing a Docker registry requiring authentication. | All |
+| `DOCKER_PASSWORD`              | `$CI_REGISTRY_PASSWORD` | Password for accessing a Docker registry requiring authentication. The default is only set if `$DOCKER_IMAGE` resides at [`$CI_REGISTRY`](../../../ci/variables/predefined_variables.md). | All |
+| `DOCKER_USER`                  | `$CI_REGISTRY_USER` | Username for accessing a Docker registry requiring authentication. The default is only set if `$DOCKER_IMAGE` resides at [`$CI_REGISTRY`](../../../ci/variables/predefined_variables.md). | All |
 | `DOCKERFILE_PATH`              | `Dockerfile`  | The path to the `Dockerfile` to use for generating remediations. By default, the scanner looks for a file named `Dockerfile` in the root directory of the project. You should configure this variable only if your `Dockerfile` is in a non-standard location, such as a subdirectory. See [Solutions for vulnerabilities](#solutions-for-vulnerabilities-auto-remediation) for more details. | All |
 | `SECURE_LOG_LEVEL`             | `info`        | Set the minimum logging level. Messages of this logging level or higher are output. From highest to lowest severity, the logging levels are: `fatal`, `error`, `warn`, `info`, `debug`. [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/10880) in GitLab 13.1. | All |