libnetwork/resolvconf: Build: decorate error for invalid nameservers

Using the same prefix as is used in `Sandbox.loadResolvConf`, but omiting
the value, as it's already part of the error message;
829b695375/libnetwork/sandbox_dns_unix.go (L258-L261)

Unfortunately, `netip.ParseAddr` returns a non-exported (`parseAddrError`)
error-type; https://cs.opensource.google/go/go/+/refs/tags/go1.24.3:src/net/netip/netip.go;l=115

So we don't have the option to omit the `` from the error-message, and to
take the underlying `msg` field;
https://cs.opensource.google/go/go/+/refs/tags/go1.24.3:src/net/netip/netip.go;l=141-153

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/resolvconf/resolvconf.go

@@ -3,6 +3,7 @@ package resolvconf
 
 import (
 	"bytes"
+	"fmt"
 	"net/netip"
 	"os"
 
@@ -130,7 +131,7 @@ func Build(path string, nameservers, dnsSearch, dnsOptions []string) (*File, err
 	for _, addr := range nameservers {
 		ipAddr, err := netip.ParseAddr(addr)
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("bad nameserver address: %w", err)
 		}
 		ns = append(ns, ipAddr)
 	}

libnetwork/resolvconf/resolvconf_unix_test.go

@@ -349,7 +349,7 @@ search search1
 		{
 			doc:         "invalid nameserver",
 			nameServers: []string{"resolver.example.com"},
-			expErr:      `ParseAddr("resolver.example.com"): unexpected character (at "resolver.example.com")`,
+			expErr:      `bad nameserver address: ParseAddr("resolver.example.com"): unexpected character (at "resolver.example.com")`,
 		},
 	}