[setup] Docker runtime support

[tsibley]

With the new nextstrain setup command that will soon land as part of #218, the framework is in place for runner modules to automatically set up their runtime. It appears we could use the official "command-line install" instructions for Docker, which seem intended for unattended installs like setup, as guides for implementing nextstrain setup docker.

• https://docs.docker.com/desktop/install/mac-install/#install-from-the-command-line
• https://docs.docker.com/desktop/install/windows-install/#install-from-the-command-line

Our own existing manual install instructions for Docker will be useful to cross-reference too.

Implementing this would be an exercise in attention to details, robustness in the face of different people's machines, and error handling, coupled with a lot of testing across platforms. But I think it'd be worth it, and greatly simplify our overall install instructions!

[tsibley]

Since I was setting up the Docker runtime on a new Mac today for troubleshooting, I took the opportunity to try and do it in the terminal, as Nextstrain CLI might. My notes follow.

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Download https://desktop.docker.com/mac/main/arm64/Docker.dmg

Install it:

hdiutil attach Docker.dmg
/Volumes/Docker/Docker.app/Contents/MacOS/install
hdiutil detach /Volumes/Docker

Then start the GUI app, which will prompt for privilege escalation to complete setup:

open /Applications/Docker.app

It takes a few seconds for the daemon to fully start after privileged setup is
complete.

On first docker run, another permission needed to be granted for access to
mount the host filesystem (at least in my case with ~/Downloads/).