Avoid kernel too old error when building ROS

.drone.yml

@@ -5,4 +5,4 @@ pipeline:
     volumes:
     - /var/run/docker.sock:/var/run/docker.sock
     commands:
-    - dapper ci
+    - KERNEL_CHECK=0 dapper ci

Dockerfile.dapper

@@ -40,7 +40,7 @@ RUN echo "Acquire::http { Proxy \"$APTPROXY\"; };" >> /etc/apt/apt.conf.d/01prox
 
 ########## Dapper Configuration #####################
 
-ENV DAPPER_ENV VERSION DEV_BUILD RUNTEST DEBUG APTPROXY ENGINE_REGISTRY_MIRROR INTEGRATION_TESTS
+ENV DAPPER_ENV VERSION DEV_BUILD RUNTEST DEBUG APTPROXY ENGINE_REGISTRY_MIRROR INTEGRATION_TESTS KERNEL_CHECK
 ENV DAPPER_DOCKER_SOCKET true
 ENV DAPPER_SOURCE /go/src/github.com/rancher/os
 ENV DAPPER_OUTPUT ./bin ./dist ./build/initrd ./build/kernel

scripts/build

@@ -1,6 +1,21 @@
 #!/bin/bash
 set -e
 
+version_gt() {
+    test "$(printf '%s\n' "$@" | sort -V | head -n 1)" != "$1";
+}
+
+
+if [ "$KERNEL_CHECK" != "0" ]; then
+    echo "Host Kernel: $(uname -r)"
+    echo "RancherOS Kernel: $KERNEL_VERSION"
+
+    if ! version_gt $(uname -r) $KERNEL_VERSION; then
+        echo "Your host kernel must be greater than RancherOS KERNEL_VERSION"
+        exit 1
+    fi
+fi
+
 echo -------------------------./build-target
 $(dirname $0)/build-target
 echo -------------------------./build-host