As part of my test setup, the setup script needs to append a line to /etc/hosts

Doing sudo echo "127.0.0.1 foo.bar" >> /etc/hosts results in the following error when executed inside a container

/bin/sh: /etc/hosts: Read-only file system

Googled the error and based on a suggestion from superuser.com issued the command sudo -- sh -c "echo test >> /etc/hosts". But it results in the same error

vi /etc/hosts opens the file in read-only mode. Changing permission doesn't work as well.

bash-3.2# chmod a+w /etc/hosts
chmod: changing permissions of `/etc/hosts': Read-only file system

Any suggestions?