reduce one layer in docker build

Closes BMK-74

build-executor/generate_Dockerfile.sh

@@ -35,8 +35,7 @@ function generate_Dockerfile(){
 	-e "s@__insert_bmkdescription__@$bmkdescription@g" \
 	-e "s@__insert_bmkexe__@$bmkexe@g" > $DOCKERFILE
 
-    echo "RUN tar -cf /tmp/checksum.tar /bmk /cvmfs" >> $DOCKERFILE
-    echo "RUN CHECKSUM=\`md5sum /tmp/checksum.tar| cut -f1 -d\" \"\`; rm /tmp/checksum.tar; echo '{\"version\":\"$DOCKER_IMAGE_TAG\",\"description\":\"$bmkdescription\",\"checksum\":\"'\$CHECKSUM'\"}' > /bmk/$bmkdir/version" >> $DOCKERFILE
+    echo "RUN tar -cf /tmp/checksum.tar /bmk /cvmfs && CHECKSUM=\`md5sum /tmp/checksum.tar| cut -f1 -d\" \"\` && rm /tmp/checksum.tar && echo '{\"version\":\"$DOCKER_IMAGE_TAG\",\"description\":\"$bmkdescription\",\"checksum\":\"'\$CHECKSUM'\"}' > /bmk/$bmkdir/version" >> $DOCKERFILE
 
 
 }