docker-login - Man Page

Examples (TL;DR)

Interactively log into a registry: docker login
Log into a registry with a specific username (user will be prompted for a password): docker login --username username
Log into a registry with username and password: docker login --username username --password password server
Log into a registry with password from stdin: echo "password" | docker login --username username --password-stdin

Log in to a Docker Registry located on the specified SERVER. You can specify a URL or a hostname for the SERVER value. If you do not specify a SERVER, the command uses Docker's public registry located at https://registry-1.docker.io/ by default. To get a username/password for Docker's public registry, create an account on Docker Hub.

docker login requires user to use sudo or be root, except when:

connecting to a remote daemon, such as a docker-machine provisioned docker engine.
user is added to the docker group. This will impact the security of your system; the docker group is root equivalent. See Docker Daemon Attack Surface ⟨https://docs.docker.com/engine/security/security/#/docker-daemon-attack-surface⟩ for details.

You can log into any public or private repository for which you have credentials. When you log in, the command stores encoded credentials in $HOME/.docker/config.json on Linux or %USERPROFILE%/.docker/config.json on Windows.