Add documentation for setting up authentication with a self-hosted GitLab

Signed-off-by: mcnesium <git@mcnesium.com>

docs/guides/auth.md

@@ -210,3 +210,32 @@ The basic procedure is the same as the case of OneLogin which is mentioned above
     ````
 
 
+### GitLab (self-hosted)
+
+1. Sign in to your GitLab
+2. Navigate to the application management page at `https://your.gitlab.domain/admin/applications` (admin permissions required)
+3. Click **New application** to create a new application and fill out the registration form:
+
+![New GitLab application](images/auth/gitlab-new-application.png)
+
+4. Click **Submit**
+5. In the list of applications select **HackMD**. Leave that site open to copy the application ID and secret in the next step.
+
+![Application: HackMD](images/auth/gitlab-application-details.png)
+
+
+6. In the `docker-compose.yml` add the following environment variables to `app:` `environment:`
+
+```
+- HMD_DOMAIN=your.hackmd.domain
+- HMD_URL_ADDPORT=443
+- HMD_PROTOCOL_USESSL=true
+- HMD_GITLAB_BASEURL=https://your.gitlab.domain
+- HMD_GITLAB_CLIENTID=23462a34example99fid0943c3fde97310fb7db47fab1112
+- HMD_GITLAB_CLIENTSECRET=5532e9dexample70432secret0c37dd20ce077e6073ea9f1d6
+```
+
+7. Run `docker-compose up -d` to apply your settings.
+8. Sign in to your HackMD using your GitLab ID:
+
+![Sign in via GitLab](images/auth/gitlab-sign-in.png)