Use proc fs to access sysctl values, to be able to query whether the variables...

Use proc fs to access sysctl values, to be able to query whether the variables we want to write are exposed and writable at all.

docker-compose/elk-configure-host/Dockerfile

 FROM ubuntu:20.04
 
 # Expand VM settings as required by ELK
-CMD bash -c "if test $(sysctl -n vm.max_map_count) -lt 262144; then sysctl -w vm.max_map_count=262144; fi"
+CMD bash -c "if [ -w /proc/sys/vm/max_map_count ] && [ $(cat /proc/sys/vm/max_map_count) -lt 262144 ]; then echo 262144 > /proc/sys/vm/max_map_count; fi"