How to specify hostname for the running container?

Edit /etc/hostname is one thing for which you need ssh access inside the container. Otherwise, you can spin up the container with -h option.

To set the host and domain names:

$ docker run -h foo.bar.baz -i -t ubuntu bash
root@foo:/# hostname
foo
root@foo:/# hostname -d
bar.baz
root@foo:/# hostname -f
foo.bar.baz

Stop container and service

sudo docker stop CONTAINER_NAME
sudo service docker stop

Edit config file (JSON) [You should make backup first]

/var/lib/docker/containers/CONTAINER_ID/config.json

Replace

"Hostname":"WHATEVER"

with

"Hostname":"NEW_HOSTNAME"

Start container and service

sudo service docker start
sudo docker start CONTAINER_NAME

(Optionally you can also attach docker)

sudo docker attach CONTAINER_NAME

Details about dockers (i.e. CONTAINER_NAME, CONTAINER_ID) can be obtained by running

sudo docker ps -a

In case you use --net=host then you can't change the hostname from -h or from inside the docker.

See https://github.com/docker/docker/issues/5708

Restarting the container would be the easiest option - but you may also edit /etc/hostname and go from there.

https://evolvingweb.ca/blog/changing-docker-hostnames-namespaces describes a way to do this. Basic idea is to use docker inspect to obtain the pid of the container, then enter the uts namespace of the container via nsenter. Running hostname inside that namespace will change the hostname for the docker instance that shares that namespace.