Docker 18 and beyond
There’s now an officially-documented way to do this:
cat ~/my_password.txt | docker login --username foo --password-stdin
Docker 1.11 through Docker 17
You can pass all the arguments on the command-line:
docker login --username=$DOCKER_USER --password=$DOCKER_PASS $DOCKER_HOST
If you don’t specify DOCKER_HOST, you’ll get the main Docker repo. If you leave out any of the arguments, you’ll be prompted for that argument.
Older than 1.11
The same path as just above, except that you need to also pass an --email flag. The contents of this are not actually checked, so anything is fine:
docker login --username=$DOCKER_USER --password=$DOCKER_PASS $DOCKER_HOST --email whale@docker.com