[.NET Core 2.0] Use RHEL/CentOS container image

[omajid]

Can we use an open source build of .NET Core 2.0 here:

fabric8-jenkinsfile-library/dotnet/ReleaseAndStage/Jenkinsfile

Line 12 in d78aec4

writeFile file: 'Dockerfile', text: 'FROM microsoft/dotnet:onbuild'

?

The Microsoft container image is built on top of Debian and Windows and uses their binary bits. Red Hat has a s2i-compatible container image called dotnet/dotnet-20-rhel7 and a runtime-only container image called dotnet/dotnet-20-runtime-rhel7. Equivalent builds from CentOS should be available soon.