The docker attach command attaches the running container and it is very helpful when you want to see what is written in stdout in real time:

$ sudo docker run -d --name=newtest alpine /bin/sh -c "while true; do sleep 2; df -h; done"
Unable to find image 'alpine:latest' locally
latest: Pulling from library/alpine
3690ec4760f9: Pull complete 
Digest: sha256:1354db23ff5478120c980eca1611a51c9f2b88b61f24283ee8200bf9a54f2e5c
1825927d488bef7328a26556cfd72a54adeb3dd7deafb35e317de31e60c25d67
$ sudo docker attach newtest
Filesystem                Size      Used Available Use% Mounted on
none                      7.7G      3.2G      4.1G  44% /
tmpfs                   496.2M         0    496.2M   0% /dev
tmpfs                   496.2M         0    496.2M   0% /sys/fs/cgroup
/dev/xvda1                7.7G      3.2G      4.1G  44% /etc/resolv.conf
/dev/xvda1                7.7G      3.2G      4.1G  44% /etc/hostname
/dev/xvda1                7.7G      3.2G      4.1G  44% /etc/hosts
shm                      64.0M         0     64.0M   0% /dev/shm
tmpfs                   496.2M         0    496.2M   0% /proc/sched_debug
Filesystem                Size      Used Available Use% Mounted on
none                      7.7G      3.2G      4.1G  44% /
tmpfs                   496.2M         0    496.2M   0% /dev

By default, this command attaches stdin and proxies signals to the remote process. Options are available to control both of these behaviors. To detach from the process, use the default Ctrl + C sequence.