Config driven install

[vyadh]

This PR does a few things, mostly in pursuit of #380:

1. Installs all tools required by the runner itself (at least, as far as I can determine)
2. Installs all optional tools driven from a config file
   a) apt installs are just a concatenated package list
   b) script-based tools it takes the function name to install each tool from the config
3. Takes the uid and gid from a config file, which helps matching the runner user against host user configuration (since we have a build config file now anyway...)

I took the opportunity to split out docker-cli from docker-ce. The main reason being that if you're delegating docker builds to the host runner, you only need the cli. One extra apt install but I doubt this has much impact on the build time.

[myoung34]

Revert the changes to README.md
I've captured them in a doc to put on the wiki instead of the README

[myoung34]

Cool work, thanks! I'll update the wiki

[vyadh]

Thanks for merging! This is going to be super useful to keep in sync with updates but retain a minimal image.

[myoung34]

https://github.com/myoung34/docker-github-actions-runner/wiki/Usage#modifications