build: create the pod with its port mappings if missing

Instead of erroring when the owpengram pod is absent, build.sh now creates it
with the MTProto (2398), admin (2600), extra TCP (2400) and RTC/UDP port
mappings.

build.sh

@@ -29,9 +29,14 @@ if [ "${NO_DEPLOY:-0}" = "1" ]; then
 fi
 
 if ! podman pod exists "$POD"; then
-  echo "error: pod '$POD' does not exist - create it first, e.g.:" >&2
-  echo "  podman pod create --name $POD -p 127.0.0.1:2401:2401" >&2
-  exit 1
+  echo "pod '$POD' does not exist - creating it"
+  podman pod create --name "$POD" \
+    -p 2398:2398 \
+    -p 127.0.0.1:2600:2600 \
+    -p 2400:2400 \
+    -p 12399:12399/udp \
+    -p 12400:12400/udp \
+    -p 12500-12999:12500-12999/udp
 fi
 if [ ! -f .env ]; then
   echo "error: .env not found (containers are created with --env-file .env)" >&2