Add Pipeline

[Sett17]

This adds a showcase pipeline using docker and docker-compose.

This modifies the PeekabooAV Dockerfile, to result in a smaller image than before.

And then adds other services needed for a fully functional email pipeline with an antivirus system, including:

• Postfix containers (one acting as a 'sender' and one as a 'receiver')
• rspamd as the spam filtering system, and
• Cortex for Analysis of Observables (+plus a container to automatically set it up)

[michaelweiser]

@sett17 : Thanks again for your great work on this! Feel free to jump into the discussion on #76 as well.