EXTENSION_PRIORITY env var is not respected by Docker container

version: '2.0'
networks:   
  guacnetwork_compose:     
    driver: bridge
services:   
  guacd:     
    container_name: guacd_compose
    image: guacamole/guacd
    networks:       
      guacnetwork_compose: 
    restart: always
    volumes:       
      - /home/dockeruser/guacamole-data/drive:/drive:rw
      - /home/dockeruser/guacamole-data/record:/record:rw
    environment:       
      GUACD_LOG_LEVEL: debug
  guacamole:     
    container_name: guacamole_compose
    depends_on: 
      - guacd
    environment:       
      GUACD_HOSTNAME: guacd
      MYSQL_HOSTNAME: *******
      MYSQL_PORT: 3306
      MYSQL_DATABASE: *******
      MYSQL_USER: *******
      MYSQL_PASSWORD: *******
      OPENID_AUTHORIZATION_ENDPOINT: *******
      OPENID_JWKS_ENDPOINT: *******
      OPENID_ISSUER: *******
      OPENID_CLIENT_ID:*******
      OPENID_REDIRECT_URI:*******
      EXTENSION_PRIORITY: "mysql,openid" 
  networks: 
    guacnetwork_compose: 
      ports: - 127.0.0.1:8089:8080/tcp 
  restart: always

// ...
# Set extension priority if specified
set_optional_property "extension-priority" "$EXTENSION_PRIORITY"

# Use api-session-timeout if specified.
if [ -n "$API_SESSION_TIMEOUT" ]; then
    associate_apisessiontimeout
fi

# Set logback level if specified
if [ -n "$LOGBACK_LEVEL" ]; then
    unzip -o -j /opt/guacamole/guacamole.war WEB-INF/classes/logback.xml -d $GUACAMOLE_HOME
    sed -i "s/level=\"info\"/level=\"$LOGBACK_LEVEL\"/" $GUACAMOLE_HOME/logback.xml
fi

#
# Finally start Guacamole (under Tomcat)
#

start_guacamole