[FIX] correct undefined variable DB_CONTAINER_NAME

Replace $DB_CONTAINER_NAME with $POSTGRES_SERVICE_NAME which is the
correct variable exported from the parent script (upgrade.sh).

DB_CONTAINER_NAME was never defined, causing the script to fail
immediately with 'set -u' enabled (unbound variable error). The
intended variable is POSTGRES_SERVICE_NAME which contains the name
of the PostgreSQL container detected at runtime.

prepare_db.sh

@@ -10,8 +10,8 @@ DB_FINALE_SERVICE="$4"
 echo "Start database preparation"
 
 # Check POSTGRES container is running
-if ! docker ps | grep -q "$DB_CONTAINER_NAME"; then
-    printf "Docker container %s is not running.\n" "$DB_CONTAINER_NAME" >&2
+if ! docker ps | grep -q "$POSTGRES_SERVICE_NAME"; then
+    printf "Docker container %s is not running.\n" "$POSTGRES_SERVICE_NAME" >&2
     exit 1
 fi