fix: enable IPv6 dual-stack for OSS, Plus, and unprivileged images

[I501307]

What This PR Does

Commit 1: OSS/Plus Images (Port 80)

File: common/etc/nginx/templates/default.conf.template
Change: Adds listen directives to template

server {
+   listen       80;
+   listen       [::]:80;
    include /etc/nginx/conf.d/gateway/server_variables.conf;

Result: OSS/Plus images listen on IPv4 and IPv6 port 80

Commit 2: Unprivileged Image (Port 8080)

File: Dockerfile.unprivileged
Change: REPLACES port 80 with 8080 (both IPv4 and IPv6)

-RUN sed -i "/^server {/a \    listen       8080;" ...
+RUN sed -i 's/listen[[:space:]]*80;/listen 8080;/g' ... \
+    && sed -i 's/listen[[:space:]]*\[::\]:80;/listen [::]:8080;/g' ... \

Result: Unprivileged image listens on IPv4 and IPv6 port 8080 ONLY

Final Configuration

OSS/Plus Containers:

server {
    listen       80;        # IPv4
    listen       [::]:80;   # IPv6
    ...
}

Unprivileged Containers:

server {
    listen       8080;      # IPv4 (replaced from 80)
    listen       [::]:8080; # IPv6 (replaced from [::]:80)
    ...
}

Why This Works

✅ Template has port 80 + IPv6 for OSS/Plus
✅ Unprivileged Dockerfile REPLACES port 80 → 8080 (no permission issues)
✅ No duplicate listen directives
✅ No privilege escalation required
✅ Fixes both #339 and #453
✅I have read the contributing guidelines.
✅I have signed the F5 Contributor License Agreement (CLA).
✅ The PR title follows the Conventional Commits specification.
✅If applicable, I have added tests that prove my fix is effective or that my feature works.
✅ If applicable, I have checked that any relevant tests pass after adding my changes.
✅I have updated any relevant documentation (e.g. README.md).

Issues Resolved

• Fixes Unprivileged Docker image does not listen on IPv6 #339 (Unprivileged Docker image does not listen on IPv6)
• Fixes IPv6 enabler in /docker-entrypoint.d is broken #453 (IPv6 enabler in /docker-entrypoint.d is broken)

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[I501307]

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