update with GITLAB_HOST variable for different gitlab domain

jupyterhub-rstudio-gitlab/.env

 # Name of our Docker Compose project
 COMPOSE_PROJECT_NAME=rocker_binder_gitlab
 OAUTH_CALLBACK_URL= "https://<jupyterhub_server>/hub/oauth_callback" # the same link has to be written in the gitlab application form in section URI
+GITLAB_HOST="https://gitlab.com" # or simply "gitlab.example.com". If not defined, default is https://gitlab.com.
 GITLAB_CLIENT_ID="xxxxxxxxxxxxxxxxxxxxxxxxxxxxx" # given by gitlab application
 GITLAB_CLIENT_SECRET="xxxxxxxxxxxxxxxxxxxxxxxxxxxxx" # given by gitlab application
+

jupyterhub-rstudio-gitlab/docker-compose.yml

@@ -13,6 +13,7 @@ services:
       DOCKER_JUPYTER_IMAGE: rstudio_img
       DOCKER_NETWORK_NAME: ${COMPOSE_PROJECT_NAME}_default
       HUB_IP: ${COMPOSE_PROJECT_NAME}_hub
+      GITLAB_HOST: ${GITLAB_HOST} # if not specified it will be gitlab.com
       GITLAB_CLIENT_ID: ${GITLAB_CLIENT_ID}
       GITLAB_CLIENT_SECRET: ${GITLAB_CLIENT_SECRET}
       OAUTH_CALLBACK_URL: ${OAUTH_CALLBACK_URL}