Fix Arch sysctl compatibility issue

- `/etc/sysctl.conf` is no longer used
- Thanks, `systemd`!  (╯°□°)╯︵ ┻━┻
Don't bind interfaces in dnsmasq

- cumbersome to add/remove interfaces (e.g., DNS for Docker)
Switch to socket activation for Docker

- also adds a `docker` group
- Explanation: http://0pointer.de/blog/projects/socket-activation.html
- Configuration: https://github.com/docker/docker/blob/660eadae3d56195e05da170593af50f0d7752706/contrib/init/systemd/docker.service
Fix for newer Docker versions

- old versions needed `docker` and `dockerinit`
- new versions just have `docker`
Fix URL for dotfiles repo
Add Avahi (zeroconf, mDNS) to all Arch hosts

- allows resolving `.local` addresses