Make t3c debugging shim inherit the CDN in a Box environment

[zrhoffman]

#7647 fixes an issue introduced by switching from centos:8 to rockylinux:8 in CDN in a Box, which made cron jobs no longer inherit the environment, which made t3c fail in CDN in a Box when using debug images.

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Which Traffic Control components are affected by this PR?

• CDN in a Box

What is the best way to verify this PR?

1. Build and start CDN in a Box using the debug images and include the readiness service:

   docker-compose -f docker-compose.yml -f optional/docker-compose.debugging.yml -f docker-compose.readiness.yml up --build -d

2. Verify that the readiness container exits successfully (which means that t3c runs as expected):

3. docker-compose -f docker-compose.yml -f optional/docker-compose.debugging.yml -f docker-compose.readiness.yml logs -f trafficrouter readiness

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)