Install libelf-dev for CONFIG_STACK_VALIDATION

tools/kernel-factory/Ubuntu/18.04/Dockerfile

@@ -1,4 +1,4 @@
 FROM ubuntu:18.04
 
 RUN apt-get update
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y linux-image-4*-generic linux-headers-*-generic build-essential wget git
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y linux-image-4*-generic linux-headers-*-generic build-essential wget git libelf-dev