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

[graphaelli]

Suggested change

	## Pre-requisites
	We'll be using the Terminal app to run commands.
	You can already launch it by hitting the `command+space` keys then searching for `terminal`.
	### Install Docker and configure it
	1. Download and install [Docker Desktop](https://docs.docker.com/desktop/install/mac-install/) for Mac.
	You'll get a warning saying that Docker Desktop needs privileged access - you will need to enter your computer password when prompted.
	You'll see a "Docker Subscription Service Agreement" screen - accept it.
	2. Start Docker and create an account
	Open the Docker app, and follow the steps to create an account.

This is not zat specific so I'd prefer we just remove it altogether

[graphaelli]

Suggested change

	### Log into Elastic container registry
	We're now going to log into Elastic container registry, where we'll download zat.
	Visit [https://docker-auth.elastic.co](https://docker-auth.elastic.co) - click on the `Login to @elastic with GitHub` button.
	You may get redirect to an Okta log-in screen, but eventually will see a page as below:
	
	Copy the full line, after the `$` sign and paste it into the Terminal window and press enter.
	You should see a `Login Succeeded` message.

This is a high barrier too, why not just docker hub since this is all public anyway?

[graphaelli]

Lots of overlap here with the README but these are excellent and I love the screenshots - I'd be happy to merge this in and consolidate the two docs in a follow up.