I tried several different ways to create a simple text file inside a Docker Volume. The following worked:

docker volume create --name=mysql-configuration

docker run \
  --rm=true \
  --volume=mysql-configuration:/data \
  ubuntu \
  sh -c 'echo "[mysqld]\nmax_connections=100" > /data/mysql-configuration.cnf'

docker run \
  --rm=true \
  --volume=mysql-configuration:/data \
  ubuntu \
  cat /data/mysql-configuration.cnf

I hope that the Docker Volume system evolves to act like Hadoop’s. It would be nice to have put and get functionality.