Add dependency proxy authentication example

doc/ci/docker/using_kaniko.md

@@ -74,6 +74,13 @@ build:
     - if: $CI_COMMIT_TAG
 ```
 
+If you authenticate against the [Dependency Proxy](../../user/packages/dependency_proxy/index.md#authenticate-within-cicd),
+you must add the corresponding CI/CD variables for authentication to the `config.json` file:
+
+```yaml
+- echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(printf "%s:%s" "${CI_REGISTRY_USER}" "${CI_REGISTRY_PASSWORD}" | base64 | tr -d '\n')\"},\"$CI_DEPENDENCY_PROXY_SERVER\":{\"auth\":\"$(printf "%s:%s" ${CI_DEPENDENCY_PROXY_USER} "${CI_DEPENDENCY_PROXY_PASSWORD}" | base64 | tr -d '\n')\"}}}" > /kaniko/.docker/config.json
+```
+
 ### Building an image with kaniko behind a proxy
 
 If you use a custom GitLab Runner behind an http(s) proxy, kaniko needs to be set