The same Docker image cannot work both on IPv4-only and IPv6 networks

Deployments with IPv6 require forking the image or overwriting `nginx.conf`.

When listening on `listen [::]:80;`, @pando85 reported in #189 that IPv4-only Docker installs do not work as Nginx is trying to bind on IPv6 and fails.

However, when binding on `listen 80` in this repo's config, it appears that Nginx only binds on IPv4 and is not reachable over IPv6.

The issue seems more complex than I expected as the official Nginx Docker image itself is [using an entrypoing bash script](https://github.com/nginxinc/docker-nginx/blob/2ef3fa66f2a434cd5e44e35a02f4ac502cf50808/stable/buster/10-listen-on-ipv6-by-default.sh#L57) to replace the `listen 80` line before running Nginx:
```bash
# enable ipv6 on default.conf listen sockets
sed -i -E 's,listen 80;,listen 80;\n listen [::]:80;,' /$DEFAULT_CONF_FILE
```

Any idea how to solve this and have the same image on

#### Related links:
- https://nginx.org/en/docs/http/ngx_http_core_module.html#listen
- https://github.com/nginxinc/docker-nginx/blob/2ef3fa66f2a434cd5e44e35a02f4ac502cf50808/stable/buster/10-listen-on-ipv6-by-default.sh#L57
- https://github.com/wallabag/docker/pull/173
- https://github.com/wallabag/docker/pull/189