Use Docker secrets instead of plain text environment variables

[pedropombeiro]

 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "FTP_PASSWORD") (line 64)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "TELEGRAM_TOKEN") (line 68)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "SSH_PASSWORD") (line 58)

We should pass *_FILE environment variables instead of ones passed in plain text. The script would then read the file contents at startup, avoiding exposing the passwords in the environment variables.

[PhilippMundhenk]

Could be the (default) alternative. Some of these might not be particularly secret and passing as file might be "overkill". But we certainly should support the option to do better...