Add RHEL 7 to CI

[omajid]

For every PR, we run the entire test suite (with some exceptions) on all distributions that we care about to make sure the tests are all passing (or failing, where appropriate). Unfortunately, there's one distribution - RHEL 7 - that's not tested. It's also quite a bit of a unique platform with oldre/different packages, commands and paths.

We should add it to our CI matrix

See https://github.com/redhat-developer/dotnet-regular-tests/pull/271#issuecomment-1630246506

[omajid]

Unfortunately, there's no simple/public way to get RHEL 7 + .NET from RHEL 7. We can get a RHEL 7 container using ubi7, but we can't install much in it.

Here's one way I am thinking we could make it work:

• Create a container inside the Red Hat firewall, installing all the packages we need for tests, specially .NET 6. This could use internal package repositories or official packages from repositories that require subscription credentials
• Upload the container to a registry that's not public/requires authentication
• Add registry credentials to this repo
• Update GitHub Actions steps to pull down that container using the credentials
• Use the container to run tests

[omajid]

With .NET on RHEL 7 reaching its EOL in a few months (see https://access.redhat.com/support/policy/updates/net-core), I don't think implementing this is a high priority.