By skyforbes 
TL;DR — Add a DNS entry to your Docker Engine. I ended up using Google & Cloudflare
"dns" : ["8.8.8.8","1.1.1.1"]
THE PROBLEM
When I was trying to have my Docker build install a PHP Pecl package I ran into this issue that blocked my build.
THE SOLUTION
Because I was building this on a corporate box, it seems the DNS on the VPN (Cisco Umbrella) was a bit odd and missing generic DNS servers for Pecl’s resolution. With this fix, I was able to successfully build my container with the Pecl package inside.
"dns" : ["8.8.8.8","1.1.1.1"]
Happy Coding!