fix: Minimum fix to not always write the config file

[xaviergrandmougin]

Proposed changes

The listen-on-ipv6-by-default script is always trying to modify the /etc/nginx/conf.d/default.conf file, even if the configuration is already correct. This leads to an error in the logs when using a read-only file system.

No attempt to modify /etc/nginx/conf.d/default.conf should be done if the configuration is correct, and this issue is fixed in this short PR.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have run the update.sh script and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• I have tested that the NGINX Docker unprivileged image builds and runs correctly on all supported architectures on an unprivileged environment (check out the README for more details)
• I have updated any relevant documentation (README.md)

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[xaviergrandmougin]

I have hereby read the F5 CLA and agree to its terms

[xaviergrandmougin]

The title says "minimum fix", because the grep pattern could be upgraded to something like listen[[:space:]][[:space:]]*\[::]\:8080;.

[alessfg]

Thanks for the fix!

[alessfg]

Just as a brief FYI, I am waiting until nginx/docker-nginx#994 gets merged and then I will merge this PR :)