Failures to write to host from non-root processes

[sadovnikov]

Start minimesos cluster and install MySql using

{
  "id": "mysql",
  "env": {
    "MYSQL_ROOT_PASSWORD": "RNwULK1oySla",
    "MYSQL_USER": "wpdb",
    "MYSQL_PASSWORD": "e2AtRUUlgLel",
    "MYSQL_DATABASE": "wp"
  },
  "container": {
    "type": "DOCKER",
    "docker": {
      "network": "BRIDGE",
      "image": "mysql:5.7.12"
    },
    "volumes": [
      {
        "hostPath": "${MINIMESOS_HOST_DIR}/data/mysql",
        "containerPath": "/var/lib/mysql",
        "mode": "RW"
      }
    ]
  },
  "cpus": 1,
  "mem": 512,
  "instances": 1,
  "constraints": [
    ["hostname", "UNIQUE"]
  ]
}

Container starts, but mysql fails to create databases - unable to write to disk.
To work-around this, remove ${MINIMESOS_HOST_DIR} from hostPath. /data/mysql directory will get created in docker-machine, and mysql is able to write

[sadovnikov]

Possibly, has the same cause as #426

[sashkachan]

I think it's reasonable to make it a requirement to run docker with user namespaces enabled.
Then root account in a container will always map to the user who's running the container.
As long as the containers run under root, writing something in mapped volumes will result in files owned by root. Only container processes run under root and root account can make changes to these files then.

Also described here #332