As long as you know what you're doing, there's no problem building as root.
Maybe. But there were multiple cases in the past where this didn't work as plan :s Better build as user and avoid one possible problem.
The main problem here is that sources are now in folder moby-1.13.1 (content has been changed even if filename is the same - happen in the past for other packages). That's why docker package.mk needs new line
Time to add SHA256 checksums in our build system. Who will start on that (I have free time)?