This is the first time I remember seeing any problems with Digital Ocean in the couple years I’ve been dealing with them. They use Google’s DNS servers by default, and Google is rate-limiting them. I don’t know whether this is in all data centers, but I know it’s in NYC2 (where I typically install for my clients). digitaloceanstatus.com has some info at the top of the page.
If you’re having strange problems and/or rebuilds are failing, this is likely why.
A fix (recommended by Digital Ocean) is to edit
nano /etc/resolv.conf) and replace the name server lines that are there with OpenDNS’s servers.
nameserver 220.127.116.11 nameserver 18.104.22.168
On the next reboot (which for my install customers will be soon after the next security update that requires a reboot),
/etc/resolv.conf will be re-written with Google’s name servers.