CentOS/RHEL: Use dnf if available

[kajinamit]

... instead of yum. The yum command is no longer available by default in recent versions.

Closes #394

[thaJeztah]

LGTM - looks like a sane approach, thanks!

[thaJeztah]

@neersighted PTAL

FWIW; one difference in dnf that I'm aware of is that yum install also updates packages already installed, whereas dnf install doesn't.

This script is not designed to do updates of existing installations, but we're aware of users trying to use it for that purpose. Wondering if we should account for that 🤔

[neersighted]

It looks like --best should trigger the 'update the packages specified in the command line' behavior that Yum had; I agree we should probably specify it.

[thaJeztah]

To be fair; we print a warning (and add a delay) if we detect docker was already installed to warn the user "this script might not work for you", but there's a bit of a blurry line there. Would be nice if the script worked for upgrades as well (as we know people try either way), but not sure if we can deliver any promises on that (without a full evaluation of all parts of the script).

[neersighted]

I agree it's best effort; still, it's somewhat trivial to add the correct flag here. I'll amend this PR.

[thaJeztah]

LGTM