I had a problem with Docker on Mac OS X – nowadays it comes with xHyve VM instead of VirtualBox, and the VM uses cow2 format for it’s disk image, so it wasn’t clear how can one access files in /var/lib/docker on the VM.
Why does one even have to access the files? Well, there can be number of reasons. In my case it was Mosquitto container where Mosquitto MQTT broker is a PID 1 process, and when it’s not running – the container isn’t running either. So while changing a config file for Mosquitto I had to do some experimenting, which caused Mosquitto to fail on startup due to bad configuration.
As you can probably guess, it wasn’t possible to fix Mosquitto config otherwise than via /var/lib/docker, because I could not start the container anymore. So it was either this, or start from scratch with new container.
Anyhow, I did not find any working way to mount cow2 image, but I have found a solution on Docker forums to get terminal into xHyve VM when docker was started:
The username is root with no password.
Thanks to Rohinton Kazak for posting that answer.