diff --git a/README.md b/README.md
index 9258bbe..7ad036f 100644
--- a/README.md
+++ b/README.md
@@ -64,20 +64,28 @@ mkdir -p ./volumes/web/cert
 cp PATH-TO-CERT.PEM ./volumes/web/cert/cert.pem
 cp PATH-TO-KEY.PEM ./volumes/web/cert/key-no-password.pem
 ```
+#### 5.2 Configure SSO with GitLab
+If you are looking for SSO with GitLab and you use self signed certificate you have to add the PKI chain of your authority in app because Alpine doesn't know him. This is required to avoid **Token request failed: certificate signed by unknown authority**
 
-#### 5.2 Let's Encrypt
+For that uncomment this line :
+```
+# - ${GITLAB_PKI_CHAIN_PATH}:/etc/ssl/certs/pki_chain.pem:ro
+```
+
+#### 5.3 Let's Encrypt
 For using Let's Encrypt you can use this Bash script located in scripts/issue-certificate.sh (or follow the steps in docs/issuing-letsencrypt-certificate.md). Make sure to adjust `mm.example.com` to match your domain configured in step 2.
 ```
 bash scripts/issue-certificate.sh -d mm.example.com -o ${PWD}/certs
 ```
 Otherwise please consult the Certbot [documentation](https://certbot.eff.org/instructions) on how to issue a standalone certificate and ensure the paths to the certificate and key are correctly set in your *.env*.
 
-#### 5.3 Adjusting the `.env` file.
-Once you've completed 5.1 or 5.2 you'll need to adjust the `.env` file accordingly. With 5.1 verify the first two lines below are uncommented in the `.env` file, with 5.2 comment out the first two lines and uncomment the last two lines.
+#### 5.4 Adjusting the `.env` file.
+Once you've completed 5.1 or 5.2 or 5.3 you'll need to adjust the `.env` file accordingly. With 5.1 verify the first two lines below are uncommented in the `.env` file, with 5.2 uncomment the third line and put the correct path for your pki chain, with 5.3 comment out the first two lines and uncomment the last two lines.
 
 ```
 CERT_PATH=./volumes/web/cert/cert.pem
 KEY_PATH=./volumes/web/cert/key-no-password.pem
+#GITLAB_PKI_CHAIN_PATH=<path_to_your_gitlab_pki>/pki_chain.pem
 #CERT_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/fullchain.pem
 #KEY_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/privkey.pem
 ```
diff --git a/docker-compose.nginx.yml b/docker-compose.nginx.yml
index a709d55..e91bf4a 100644
--- a/docker-compose.nginx.yml
+++ b/docker-compose.nginx.yml
@@ -21,9 +21,13 @@ services:
       - ${CERT_PATH}:/cert.pem:ro
       - ${KEY_PATH}:/key.pem:ro
       - shared-webroot:/usr/share/nginx/html
+      # When you want to use SSO with GitLab, you have to add the cert pki chain of GitLab inside Alpine
+      # to avoid Token request failed: certificate signed by unknown authority 
+      # (link: https://github.com/mattermost/mattermost-server/issues/13059 and https://github.com/mattermost/docker/issues/34)
+      # - ${GITLAB_PKI_CHAIN_PATH}:/etc/ssl/certs/pki_chain.pem:ro
     environment:
       # timezone inside container
-      - TZ
+      - TZ: ${TZ}
     ports:
       - ${HTTPS_PORT}:443
       - ${HTTP_PORT}:80
diff --git a/docker-compose.yml b/docker-compose.yml
index 2721f5e..f1d27f6 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -18,7 +18,7 @@ services:
       - ${POSTGRES_DATA_PATH}:/var/lib/postgresql/data
     environment:
       # timezone inside container
-      - TZ
+      - TZ: ${TZ}
 
       # necessary Postgres options/variables
       - POSTGRES_USER
@@ -45,7 +45,7 @@ services:
       - ${MATTERMOST_CLIENT_PLUGINS_PATH}:/mattermost/client/plugins:rw
     environment:
       # timezone inside container
-      - TZ
+      - TZ: ${TZ}
 
       # necessary Mattermost options/variables (see env.example)
       - MM_SQLSETTINGS_DRIVERNAME
diff --git a/env.example b/env.example
index 3dc0e01..f2a58a5 100644
--- a/env.example
+++ b/env.example
@@ -37,6 +37,7 @@ NGINX_DHPARAMS_FILE=./nginx/dhparams4096.pem
 
 CERT_PATH=./volumes/web/cert/cert.pem
 KEY_PATH=./volumes/web/cert/key-no-password.pem
+#GITLAB_PKI_CHAIN_PATH=<path_to_your_gitlab_pki>/pki_chain.pem
 #CERT_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/fullchain.pem
 #KEY_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/privkey.pem