Refactor exec of child process to trap, spawn, signal, and wait pattern

[felddy]

🗣 Description

This PR changes the way the child processes are managed by the entrypoint script.
Specifically it backs out some of the changes from #822

Instead of execing the launcher.sh it will now be spawned into the background and waited on.
TERM signals will be trapped and forwarded to the child while waiting.
This allows the DDOS bucket protection code to execute if needed.

It also reduces the sleep interval in the infinite loop to allow container shutdown to be more responsive
when sleeping.

See:

• Properly utilize exec in helper scripts #822
• Improve release caching and prevent download loops #307

Closes #813

💭 Motivation and context

I realized that the DDOS protection in the entrypoint would not execute after the exec call.
I expect that there are deployments that have disabled caching that may still need this mitigation.

🧪 Testing

I tested locally that the signals are properly handled by the child processes.
I disable the CONTAINER_CACHE and killed the node process to verify that the entrypoint would
execute the infinite sleep.
Also CI.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• All new and existing tests pass.