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

What are the ways get the docker host's hostname from inside a container running on that host besides using environment variables? I know I can pass the hostname as an environment variable to the container at container creation time. I'm wondering how I can look it up at run time.

foo.example.com (docker host)
  bar (docker container)

Is there a way for container bar running in docker host foo.example.com to get "foo.example.com"?

Edit to add use case:

The container will create an SRV record for service discovery of the form

_service._proto.name. TTL class SRV priority weight port target.
-----------------------------------------------------------------
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo.example.com.

where 20003 is a dynamically allocated port on the docker host for a service listening on some fixed port in bar (docker handles the mapping from host port to container port).

My container will run a health check to make sure it has successfully created that SRV record as there will be many other bar containers on other docker hosts that also create their own SRV records.

_service._proto.name. TTL class SRV priority weight port target.
-----------------------------------------------------------------
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo.example.com. <--
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo2.example.com.
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo3.example.com.

The health check will loop through the SRV records looking for the first one above and thus needs to know its hostname.

aside

I'm using Helios and just found out it adds an env var for me from which I can get the hostname. But I was just curious in case I was using docker without Helios.

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) ..

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.

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.

Another option that worked for me was to bind the network namespace of the host to the docker.

By adding:

docker run --net host