How to get the hostname of the docker host from inside a docker container on that host without env vars

You can easily pass it as an environment variable

docker run .. -e HOST_HOSTNAME=`hostname` ..

using

-e HOST_HOSTNAME=`hostname`

will call the hostname and use it's return as an environment variable called HOST_HOSTNAME, of course you can customize the key as you like.

note that this works on bash shell, if you using a different shell you might need to see the alternative for "backtick", for example a fish shell alternative would be

docker run .. -e HOST_HOSTNAME=(hostname) ..

You can pass in the hostname as an environment variable. You could also mount /etc so you can cat /etc/hostname. But I agree with Vitaly, this isn't the intended use case for containers IMO.

I'm adding this because it's not mentioned in any of the other answers. You can give a container a specific hostname at runtime with the -h directive.

docker run -h=my.docker.container.example.com ubuntu:latest

You can use backticks (or whatever equivalent your shell uses) to get the output of hosthame into the -h argument.

docker run -h=`hostname` ubuntu:latest

There is a caveat, the value of hostname will be taken from the host you run the command from, so if you want the hostname of a virtual machine that's running your docker container then using hostname as an argument may not be correct if you are using the host machine to execute docker commands on the virtual machine.