Synology Docker, login not possible

[accessburn]

Good morning...
I am running a Synology and wanted to run Roundcube locally in a Docker. My email provider is external.
When logging in with my external email address, I always get the response: "Login failed". Does anyone have any ideas what I could do or possibly even an executable config?

Code: [Select]

version: '2'

services:
  roundcubemail:
    image: roundcube/roundcubemail:latest
    container_name: roundcubemail
#    restart: unless-stopped
    volumes:
      - ./:/var/www/html
      - ./:/var/roundcube/db
    ports:
      - 88:80
      - 143:143
    environment:
      - ROUNDCUBEMAIL_DB_TYPE=sqlite
      - ROUNDCUBEMAIL_SKIN=elastic
      - ROUNDCUBEMAIL_DEFAULT_HOST=***.com
      - ROUNDCUBEMAIL_SMTP_SERVER=***.com
      - ROUNDCUBEMAIL_DEFAULT_PORT=143
      - SMTP_USER=***@***.com
      - SMTP_PASS=***

Edit: changed dockerfile!
Edit: I have tested the same file locally in a VM and not on the NAS, where it runs without any problems. It seems to be due to the Synology. Any ideas?