make ipv6 in nginx.conf configurable

[avk999]

Issue Summary
On machines w/o ipv6 support the container fails:

09:05:32 [emerg] 15#15: socket() [::]:8080 failed (97: Address family not supported by protocol)                                  nginx: [emerg] socket() [::]:8080 failed (97: Address family not supported by protocol)                                                       nginx: configuration file /app/nginx.conf test failed                                         

Environment

• OS: Linux

• API version: [eg. 6.8.1]

• Deployment: Kubernetes

• For self-hosted, WSGI environment: [eg. nginx/uwsgi, apache/mod_wsgi]

• Database: irreleveant

• Server config:
  irrelevant

To Reproduce
Steps to reproduce the behavior:

1. disable ipv6
2. run the container
3. observe failure

Expected behavior
nginx listening on ipv6 configurable

The problem is in this line:

alerta@d14274541545:/$ grep :: /app/nginx.conf.j2
        listen [::]:8080 default_server;