Add support for DOCKER_AUTH_CONFIG

mixins.yml

@@ -9,6 +9,7 @@
     - if [[ -n $SSH_PRIVATE_KEY ]]; then ssh git@${CI_SERVER_HOST}; fi
     - if [[ -n $GITLAB_ACCESS_TOKEN ]]; then composer config -g --no-plugins --no-interaction gitlab-token.${CI_SERVER_HOST} $GITLAB_ACCESS_TOKEN; fi
     - if [[ -n $GITLAB_PRIVATE_TOKEN ]]; then composer config -g --no-plugins --no-interaction gitlab-token.${CI_SERVER_HOST} $GITLAB_PRIVATE_TOKEN; fi
+    - if [[ -n $DOCKER_AUTH_CONFIG ]]; then mkdir ~/.docker; echo $DOCKER_AUTH_CONFIG > ~/.docker/config.json; fi
 
 .preparecomposerplugins:
   before_script: