Add support for vsftpd min & max ports

[NateyB]

This PR aims to allow users to run more than one container on a single host. The reasoning:

• In passive mode, vsftpd sends the data port number to the client
• The client then attempts to initiate a data connection to the port provided
• Since vsftpd is unaware of port mappings via docker because docker acts transparently, the host data ports must match the vsftpd data ports—in other words, you cannot remap the data ports on the host

Adding these environment variables allows the user to customize the range of ports used for data in the container (still subject to the requirement that the host ports match).

I tested this on my setup using podman with the following docker compose file (and FileZilla on another machine) and it worked as expected:

services:
  ftps-dev-service:
    image: localhost/ftps-dev
    container_name: ftps-dev-container
    environment:
      - BF_FTPS_VSFTPD_UID=1000
      - BF_FTPS_EXTERNAL_IP=10.0.0.240
      - BF_FTPS_VSFTPD_USER=ftp_user_user
      - BF_FTPS_VSFTPD_PASS=<REDACTED>
      - BF_FTPS_VSFTPD_ENABLE_DEBUG_LOG=1
      - BF_DEBUG=1
      - BF_FTPS_VSFTPD_MIN_PORT=30010
      - BF_FTPS_VSFTPD_MAX_PORT=30020
    ports:
      - 0.0.0.0:30000:21
      - 0.0.0.0:30001:990
      - 0.0.0.0:30010-30020:30010-30020
    volumes:
      - ./uploads:/files
      - ./ssl:/ssl

[NateyB]

Would address #148

[bfren]

Thanks I'll give it a test myself!