Docker: How to authenticate for docker push?

I would think they keep passwords off the command line for security reasons.

The way to do it is to login first then push.

https://docs.docker.com/mac/step_six/

$ docker login --username=maryatdocker --email=mary@docker.com
Password:
WARNING: login credentials saved in C:\Users\sven\.docker\config.json
Login Succeeded

Then push

$ docker push maryatdocker/docker-whale
The push refers to a repository [maryatdocker/docker-whale] (len: 1)
7d9495d03763: Image already exists
c81071adeeb5: Image successfully pushed