Base docker image on alpine and add gcc 9 build
Created by: matthew-d-jones
Base the docker image on Alpine 11, which has gcc 9.2. Makes the image smaller and faster to build for system tests.
Also adds an alpine pipeline build, which will ensure we have package binaries on the conan server for the system test image to use. Having a gcc 9.2 build already should also mean those of us who run Ubuntu can switch to Ubuntu 20.04 LTS as soon as it is released in April.
Removed CentOS-debug and Debian10 builds.