fix: update NGINX Plus Dockerfile

[alessfg]

Proposed changes

This PR refactors the NGINX Plus Dockerfile by using the NGINX Plus Dockerfile found here gist.github.com/nginx-gists/36e97fc87efb5cf0039978c8e41a34b5 as a starting point.

This PR also refactors the various ENV instructions in both the NGINX OSS and NGINX Plus Dockerfiles to use the new guidelines where the key=value pair is separated by "=" instead of a whitespace.

Checklist

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• [x] I have read the contributing guidelines.
• [x] I have signed the F5 Contributor License Agreement (CLA).
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works.
• [x] If applicable, I have checked that any relevant tests pass after adding my changes.
• [x] I have updated any relevant documentation (e.g. README.md).