Dockerize me and set me up for wider adoption

[nkammah]

This PR introduces a dockerized version of Zat, and a detailed installation guide for folks who may not have a full dev. environment setup.

Notes:

• It seems like Docker Desktop uses port 8080, so I had to use a different one to redirect requests from Zat to the host - I picked 8060
• For the launchctl configuration, while storing configs in /usr/local/etc/zat/ was really appealing, DockerDesktop, at least on mac, does not allow that location by default - so I picked $HOME/.zatconfig instead
• In this PR, I pushed the repo to my namespace in the Elastic container registry, but we can and should totally move it to your namespace Gil
• It would be very interesting to find a flow to share (maybe via vault or pastebin on demand) the Zat Slack app credentials