2 min read

Docker tips

Jan 04, 2020

Some useful tips for docker:

  1. Making shortcuts for useful commands (with flags) to free up space is one time effort compared to remembering which command Docker provides. Use alias on macOS and put them in ~/.bash_profile so they’re loaded in your terminal (I use ~/.bashrc for aliases holder, which needs to be sourced unlike .bash_profile, see my .dev/dotfiles repo for more info). Alias is auto completed in bash.

# remove just dangling images
alias docker-dangling-images="docker rmi $(docker images --filter dangling=true)"

# remove every stopped containers
alias docker-stopped-containers="docker rm $(docker ps -a -q -f status=exited)"

# remove every stopped containers, also
# all networks not used by at least one container,
# all dangling images & dangling build cache
alias dockerprune="docker system prune"
  1. Useful to modify Docker Hyperkit VM’s transparent_hugepages setting in case of redis complaining (macOS Mojave 10.14.x)

WARNING you have Transparent Huge Pages (THP) support enabled in your kernel.

see
https://github.com/docker-library/redis/issues/55#issuecomment-368346882
https://stackoverflow.com/questions/39739560/how-to-access-the-vm-created-by-dockers-hyperkit)
https://kerneltalks.com/services/what-is-huge-pages-in-linux/

screen ~/Library/Containers/com.docker.docker/Data/vms/0/tty

echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo never > /sys/kernel/mm/transparent_hugepage/defrag

# exit: Ctrl-A-K then y
  1. Docker build (or docker-compose build) tips:
  • Use multi stage build for faster build
  • Clean up build artifacts & caches on the same command line layer, each RUN line adds an immutable layer which affects final image file size.
  1. Docker-compose:
  • use $$ (double-dollar sign) when using an existing environment variable to escape/prevent docker-compose interpolating a value. Example for path defined with $PGDATA environment variable in official postgresql image:

db:
	image: postgres:9.6
	volumes:
		- ./postgres:$$PGDATA

"${VAR}" (with curly bracket inside double quotation mark) or "$VAR" syntax works for shell variable, but docker-compose (depending on which file format version) expecting "${VAR}" value from .env file (exact name .env needed, otherwise ignored) when evaluating docker-compose.yml, and issues a warning if .env file is not supplied/found:

WARNING: The PGDATA variable is not set. Defaulting to a blank string.

Read more in the docs, and be sure to check on which corresponding version you’re using.