Arch Linux was listed in SUPPORTED_OS but InstallDocker.php had no
specific handler for it, causing 'Unsupported OS' errors when trying
to add Arch Linux servers.
This adds:
- Detection of 'arch' OS type in the install flow
- New getArchDockerInstallCommand() method using pacman:
- pacman -Syyy (refresh package databases)
- pacman -S docker docker-compose (install Docker)
- systemctl start/enable docker
Fixes#4523