turn on any additional functionality beyond the ubuntu image with vsftp installed.

the entrypoint checks for the presence of this variable at startup and starts vsftpd with the customized config file.

edit the pasv_max_port entry in the customized config file before starting.

this restricts the passive port range.

see FTP_PASV_MAX_PORT.

defaults to 127.0.0.1 - this is what gets sent in the PASV response.