php zmq in docker and checking whether the C compiler works... no

 I spent 2 hours of what it lasts of my life trying to understand what was the problem on installing php's zmq extension in a docker running alpine as base image. I would like to share my experience here.

 During morning I would like to write supporting class for integrating zmq into our system and use it from inside containers where services are  supposed to run.

First I just added to Dockerfile the pecl install command, and expected it works: