Feature request: Allow setting FTP control port via environment variable

[Aejs]

Hi,

First of all, thanks for providing docker-alpine-ftp-server — it's super lightweight and works great.

Currently, the passive mode ports (MIN_PORT and MAX_PORT) and ADDRESS can be configured via environment variables, but the FTP control port (default 21) can only be changed by modifying vsftpd.conf or rebuilding the image.

Feature request:
It would be great if we could set the control port directly via an environment variable (e.g. LISTEN_PORT), similar to how MIN_PORT and MAX_PORT work.
The entrypoint could simply add -olisten_port=$LISTEN_PORT when starting vsftpd if the variable is present.

Example:

docker run -d \
  -p 2121:2121 \
  -e LISTEN_PORT=2121 \
  -e MIN_PORT=30000 \
  -e MAX_PORT=30010 \
  delfer/alpine-ftp-server