Adding FTP Support for Docker ENV for Testing and Debugging

[SirLouen]

In the same fashion as #63135, it's an adventure to debug FTP related topics with the wordpress-develop docker/env. It takes me hours just to set up, debug and get everything ready anytime I need to look into anything-ftp.

I think it's time to upgrade the developer experience. I'm going to provide a potential patch, but I think this needs a little more discussion to get to a nice idea to provide these kind of basic services that WP uses internally for testing and debugging comfortably).

In my quest of trying to add some features for testing/debugging in env/docker, here I'm providing a simple setup for an FTP server than it's used in a couple of elements in WP sites.

For now, I'm not going to add instructions because, contrarily to the Mailhog patch that I think it's 100% complete, I feel that this one needs a little more polish and I need more ideas. For now, it's a working version. I might use for testing and debugging on top of certain tickets that need FTP and then rebasing to wipe this commit.

Trac ticket: https://core.trac.wordpress.org/ticket/63172

@desrosj ​https://github.com/WordPress/wpdev-docker-images/pull/165
I've added the changes also here to adapt to the official image with these changes introduced in the wpdev-docker-images PR I sent