install-proxy: auto-detect default-route interface for IP lookup

[RobertNils]

Problem

install/nginx-proxy/install-proxy.sh discovers the server's IP by probing a hardcoded interface list:

NET_INTERFACES=( eth0 ens3 ens4)

On hosts whose primary NIC has a different name this loop never finds an IP, so NET_IP is empty and fresh-start.sh is invoked with an empty --ip-address, aborting the install:

We could not identify your IP Address, please update the .env and restart the nginx-proxy!
✘ ERROR   Invalid option for --ip-address

Ubuntu 24.04 and IONOS VPS name the primary interface ens6, which is not in the list, so the install fails out of the box there.

Fix

Detect the host's actual default-route interface with ip route and prepend it to the existing list, keeping the well-known names as a fallback:

DEFAULT_IFACE=$(ip -4 route show default 2>/dev/null | awk '{print $5; exit}')
[[ -n "$DEFAULT_IFACE" ]] && NET_INTERFACES=( "$DEFAULT_IFACE" "${NET_INTERFACES[@]}" )

• Backward compatible — hosts that already worked keep matching eth0/ens3/ens4 via the fallback list.
• Works regardless of interface naming (ens6, enp0s3, eno1, etc.).
• Also adds 2>/dev/null to ip address show to silence "Device does not exist" warnings for absent NICs.

Testing

On a fresh Ubuntu 24.04 host whose primary interface is ens6, I ran both detection blocks directly:

• Original logic leaves NET_IP empty (reproduces the abort).
• Patched logic in this PR detects ens6 via ip route and resolves NET_IP to the correct public IP.
• bash -n install-proxy.sh passes.

The same problem was hit on three separate Ubuntu 24.04 hosts; the equivalent fix (extending the interface list) was applied there to complete the install. This PR generalises it via auto-detection so no host-specific list edits are needed.