[FIX] correct return statement outside function

Replace 'return 1' with 'exit 1' in prepare_db.sh.

The 'return' statement is only valid inside functions. When used at
the script's top level, it behaves unpredictably - in some shells it
exits the script, in others it's an error. Using 'exit 1' explicitly
terminates the script with an error status, which is the intended
behavior when the PostgreSQL container is not running.

prepare_db.sh

@@ -12,7 +12,7 @@ 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
-    return 1
+    exit 1
 fi
 
 EXT_EXISTS=$(query_postgres_container "SELECT 1 FROM pg_extension WHERE extname = 'dblink'" "$DB_NAME") || exit 1