Upgrade to 2018M2S6

* Change docker version
* Validate checksum during installation

cozystack/Dockerfile

 FROM debian:stable-slim AS installer
 
-ARG COZY_VERSION=2018M2S5
+ARG COZY_VERSION=2018M2S6
 ARG NODE_VERSION=8.11.3
 
 WORKDIR /tmp

cozystack/install.sh

@@ -7,16 +7,27 @@ NODE_ARCH=""
 if [[ $ARCH =~ "x86" ]]; then
 	COZY_ARCH=amd64
 	NODE_ARCH=x64
+	SHA256="f86c88a99981ed9c64289465d68a97f991af743834981691074688d5d039e6f3"
 elif [[ $ARCH =~ "arm" ]]; then
 	COZY_ARCH=arm
 	NODE_ARCH=armv7l
+	SHA256="1c3cc78362471effc7a9fd7f52accac943ab0a009e4d31f4c4d850dffd4d444b"
 else
 	echo "Unknown arch $ARCH"
 	exit 1
 fi
 
 echo "Install cozy-stack..."
-wget -O /tmp/cozy https://github.com/cozy/cozy-stack/releases/download/${COZY_VERSION}/cozy-stack-linux-${COZY_ARCH}-${COZY_VERSION}
+cat <<EOF >/tmp/SHA256
+SHA256(/tmp/cozy)= ${SHA256}
+EOF
+
+wget -O /tmp/cozy https://github.com/cozy/cozy-stack/releases/download/${COZY_VERSION}/cozy-stack-linux-${COZY_ARCH}
+
+echo "Expected checkum                : $(cat /tmp/SHA256)"
+echo "Checksum of the downloaded file : $(sha256sum --tag /tmp/cozy)"
+sha256sum -c /tmp/SHA256
+
 chmod u+x /tmp/cozy
 
 echo "Install nodejs..."