apt Hi there and welcome. You have to create a Docker volume first as follows:
docker volume create --driver local --opt device="/some/path" --opt type=none --opt o=bind yourvolume
The command above will create a volume with the name "yourvolume" and uses the directory "/some/path" to store all data.
Now you can run the container:
docker run -dp 80:80 -v yourvolume:/app --name mysite automad/automad
You should then be able to access the installation in /some/path
on your host machine.