Ability to build packages on custom docker images

[rockdreamer]

As a maintainer of a medium sized set of packages for a company using Conan/Artifactory, I'd like to be able to use something like bincrafters/templates to build them.

Our current workflow allows using the the centos7-gcc9 and centos7-gcc10 docker images in connection with a custom profile and conan settings.yml to create packages that are compatible with Centos 7 (Booo! Hisss!) 😸

Ideally, I'd like to be able to alter the build matrix using a declarative approach with a file on the repository containing the package definitions.

This feature depends on #1382 to apply the common configuration bearing the custom settings.yml file.