fix(security): Restore ADD_PROXY_ROUTE functionality for docker NGINX

[bnevis-i]

This functionality was dropped in the Kong to NGINX change, and is being restored in order to make it easy to permanently add a custom route that survives "make clean" operations in Docker.

PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?) N/A already documented as POR

Testing Instructions

As per existing documentation, use ADD_PROXY_ROUTE on proxy-setup to add custom routes.
Verify changes to /etc/nginx/conf.d/custom-routes.inc and /etc/nginx/templates/custom-routes.inc.template

New Dependency Instructions (If applicable)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov-commenter]

Codecov Report

Merging #4412 (00e1079) into main (b9da6ae) will not change coverage.
The diff coverage is n/a.

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  Hits         4068     4068           
  Misses       5325     5325           
  Partials      347      347           

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[lenny-goodell]

LGTM