[main] rpm: minimal RHEL support (carry #342)

[thaJeztah]

• carries [main] RHEL PoC #342
• closes [main] RHEL PoC #342

Implements passing the username and password through build-secrets instead of build-args

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This is a PoC for building RHEL packages assuming that a RHEL username/password is available in the environment.

This is mostly intended for internal use by Docker, as it is assumed that the main utility is performing builds with a 'real RHEL' for enterprise customers. As such, there is currently no verification in this upstream repository.

However, when a unencumbered RHEL-equivalent distribution (e.g. Rocky) is supported, it will make sense to add verification for that distribution and generalize the 'rhel' infrastructure in this repo.

[thaJeztah]

Looks like it all works; here's from our internal pipeline;

#14 [rhel-base 1/1] RUN --mount=type=secret,id=rh-user --mount=type=secret,id=rh-pass <<-EOT (rm -f /etc/rhsm-host...)
#14 3.873 Registering to: subscription.rhsm.redhat.com:443/subscription
#14 3.873 The system has been registered with ID: <REDACTED>
#14 3.873 The registered system name is: buildkitsandbox
#14 51.52 Repository 'codeready-builder-for-rhel-8-x86_64-rpms' is enabled for this system.
#14 DONE 54.3s

[thaJeztah]

Failure of opensuse is unrelated; they have a major outage; #380 (comment)

[vvoland]

LGTM

[thaJeztah]

Thx! Bringing this one in; failure is unrelated, and everything else is green 👍