Per @Petermarcu @janvorli
As part of the plan to be able to ship .NET Core 2.0 that supports RHEL 6, we need build machines for that distro. This is necessary as RHEL 6 is super ancient w.r.t. the libraries that we depend on. So if we build on RHEL 7, which was our minimum supported distro, the binary will get dependencies on versions of some symbols that are not present on RHEL 6.

As a result, to run dotnet core on RHEL 6, it has to be built on RHEL 6, which requires the corresponding docker to be created.

RHEL 6 is NOT free, so we should create equivalent CentOS 6 docker instead. We already have CentoOS docker file available, which can be used as the starting point for the CentOS 6 one.