Update centos7 rpmpkg to use ruby 2.5

[wtgodbe]

Attempt to update the centos7 image to fix dotnet/aspnetcore#37406.

[dougbu]

I don't know whether they're better nor whether all remain compatible w/ CentOS 7 but rh-ruby26*, rh-ruby27*, and rh-ruby30 packages are available.

[mthalman]

Can you return the Dockerfile back to its original format of including multiple commands in one RUN instruction?

1. It's easier to read, especially when installing multiple packages.
2. It's more efficient for image size. Each instruction causes the file modifications to be written to a separate layer. So if you execute yum clean all in its own instruction, you end up having the files cleaned in the final container but all the files that were deleted still exist in the prior layers of the image and lead to unnecessary image bloat. You can read up more on Dockerfile best practices at https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run.

[wtgodbe]

@mthalman I separated it out to try to find which install command was failing:

Step 7/9 : RUN gem install --no-document git:1.7.0 ffi:1.12.2
---> Running in 22bbc5163ece
/bin/sh: gem: command not found

does RUN yum --enablerepo=centos-sclo-rh -y install rh-ruby25-rubygems not achieve that?

[mthalman]

@mthalman I separated it out to try to find which install command was failing:

If you're still working on debugging the Dockerfile, it would be better to mark this as a draft PR. In addition, it would be better to install Docker Desktop on your local machine and build the Dockerfile locally to diagnose issues.

Step 7/9 : RUN gem install --no-document git:1.7.0 ffi:1.12.2
---> Running in 22bbc5163ece
/bin/sh: gem: command not found

does RUN yum --enablerepo=centos-sclo-rh -y install rh-ruby25-rubygems not achieve that?

I don't have much experience in this area. Looking at the other Dockerfiles in this repo that use gem, I see they just install rubygems.

[wtgodbe]

Ok, should be working now