Add IMAP ready docker env for IMAP addon

Add a docker section for imap in:
https://github.com/nuxeo/nuxeo-tools-support-priv/tree/master/docker

The folder could be named mail as there might be other env needed, like SMTP only with fakeSMTP.

The following links are relevant to the current task:
https://hub.docker.com/r/tvial/docker-mailserver/
https://www.neroblanco.co.uk/2015/05/sending-tls-mail-to-exchange-from-the-command-line/

The docker image contains IMAP(S), SMTP(S), POP (optional), anti-virus and so on. This would allow to inject mails from the command-line and have a dedicated mail server, instead of using our own production accounts or remote test ones.

A script should be made available in order to inject mails quickly.

Steps:

1. bring up compose environement with IMAPS ready server and Nuxeo with IMAP addon installed
2. inject mails into the mail server
3. see if the creation of an account in thunderbird or mail(osx) can be scripted else provide instructions.