Adding FTP Support for Docker/Env

[SirLouen]

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

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Props sirlouen, desrosj.

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[SirLouen]

@desrosj WordPress/wpdev-docker-images#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