This file needs to be modified if you want memory stats (and mem limits) in your containers : _/etc/default/grub_ if you’re on Debian, or /boot/cmdline.txt if you’re on Raspberry pi

Test it with docker info, if you see

WARNING: No memory limit support  
WARNING: No swap limit support

then you need to modify the file.

References:

  • https://web.archive.org/web/20170625185736/https://awhitehatter.me/debian-jessie-wdocker/
  • https://stackoverflow.com/questions/45541242/docker-stats-shows-zero-memory-usage-even-for-running-containers