in_docker: Add unit test and make configurable paths

[cosmo0920]

Follows up #8087

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Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

==512080== 
==512080== HEAP SUMMARY:
==512080==     in use at exit: 0 bytes in 0 blocks
==512080==   total heap usage: 2,888 allocs, 2,888 frees, 798,713 bytes allocated
==512080== 
==512080== All heap blocks were freed -- no leaks are possible
==512080== 
==512080== For lists of detected and suppressed errors, rerun with: -s
==512080== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

==512087== 
==512087== HEAP SUMMARY:
==512087==     in use at exit: 0 bytes in 0 blocks
==512087==   total heap usage: 2,894 allocs, 2,894 frees, 806,053 bytes allocated
==512087== 
==512087== All heap blocks were freed -- no leaks are possible
==512087== 
==512087== For lists of detected and suppressed errors, rerun with: -s
==512087== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.