Improve network string construction in build_container

Refactored the build_container function to handle MAC, gateway, VLAN, and MTU parameters more robustly, allowing for both direct and preformatted input. This change ensures correct formatting and prevents duplicate parameter prefixes in the network string.

misc/build.func

@@ -2109,11 +2109,43 @@ build_container() {
   #  if [ "$VERBOSE" == "yes" ]; then set -x; fi
 
   NET_STRING="-net0 name=eth0,bridge=${BRG:-vmbr0}"
-  NET_STRING+="${MAC:+,hwaddr=$MAC}"
+
+  # MAC
+  if [[ -n "$MAC" ]]; then
+    case "$MAC" in
+    ,hwaddr=*) NET_STRING+="$MAC" ;;
+    *) NET_STRING+=",hwaddr=$MAC" ;;
+    esac
+  fi
+
+  # IP (immer zwingend, Standard dhcp)
   NET_STRING+=",ip=${NET:-dhcp}"
-  NET_STRING+="${GATE:+,gw=$GATE}"
-  NET_STRING+="${VLAN:+,tag=$VLAN}"
-  NET_STRING+="${MTU:+,mtu=$MTU}"
+
+  # Gateway
+  if [[ -n "$GATE" ]]; then
+    case "$GATE" in
+    ,gw=*) NET_STRING+="$GATE" ;;
+    *) NET_STRING+=",gw=$GATE" ;;
+    esac
+  fi
+
+  # VLAN
+  if [[ -n "$VLAN" ]]; then
+    case "$VLAN" in
+    ,tag=*) NET_STRING+="$VLAN" ;;
+    *) NET_STRING+=",tag=$VLAN" ;;
+    esac
+  fi
+
+  # MTU
+  if [[ -n "$MTU" ]]; then
+    case "$MTU" in
+    ,mtu=*) NET_STRING+="$MTU" ;;
+    *) NET_STRING+=",mtu=$MTU" ;;
+    esac
+  fi
+
+  # IPv6 Handling
   case "$IPV6_METHOD" in
   auto) NET_STRING="$NET_STRING,ip6=auto" ;;
   dhcp) NET_STRING="$NET_STRING,ip6=dhcp" ;;