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Docker Run Volume : Can't Find Directory – CentOS

I’m certain I’m going to sound like a giant idiot but I am struggle to understand how Docker Volumes work.

I have the following Dockerfile

FROM centos:7
MAINTAINER Giant Idiot

RUN yum -y update && 
    yum -y install epel-release vim && 
    rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

VOLUME /opt/my_vol

RUN echo "root:root" | chpasswd && 
    yum -y install openssh-server && 
    mkdir -p /var/run/sshd && 
    sed -i 's/PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config && 
    ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key -N ''

RUN yum clean all

EXPOSE 22

COPY supervisord.conf /tmp/
RUN yum -y install supervisor && 
    mkdir -p /etc/supervisor/conf.d && 
    mv /tmp/supervisord.conf /etc/supervisor/conf.d
CMD /usr/bin/supervisord -c /etc/supervisor/conf.d/supervisord.conf

Which includes the VOLUME command to create /opt/my_vol, when I run it I map the Volume like bellow (at least I think that’s what I’m doing)

docker run -d -p 221:22 -v my_vol:/opt/my_vol --name my_dock_r1 my_dock

When I log in to the Image I can see the /opt/my_vol directory just fine and when I do the following commands everything looks ok

docker volume ls
DRIVER      VOLUME NAME
local       my_vol

docker volume inspect my_vol
[
  {
    "Driver": "local",
    "Labels": null,
    "Mountpoint": "/mnt/sda1/var/lib/docker/volumes/my_vol/_data",
    "Name": "my_vol",
    "Options": null,
    "Scope": "local"
  }
]

Which I’m happy with (or not happy with because it could be wrong) but the Mountpoint doesn’t make sense to me.

I thought it mounts it to my file system (Windows 10) and there is no such directory. In that case does it mount it in a place I can find it on the file system or is my understanding of this wrong?

I’m using the VirtualBox version of Docker, I’m not sure if that makes any difference

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2

Answers


  1. Chosen as BEST ANSWER
    0

    It seems that the error is due to it being the VirtualBox Docker (Docker Container it's called).

    In order for the Volumes to be Mounted they have to be added to the VirtualBox settings. From the VirtualBox GUI

    Settings

    Shared Folders

    Add Folder (little Folder with Plus symbol on the far Right)

    Add the folder in the prompt

    Folder Path : C:/Some/Directory

    Folder Name : c/Some/Directory

    Auto-mount : Checked

    Make Permanent : Checked

    then Restart the VirtualBox and after that the following command works

    docker run -d -p 221:22 -v //c/VMDocker/Vol:/opt/my_vol --name my_dock_r1 my_dock

  2. 0

    If you need to directly access the content of a volume from the host, you must use bind mount syntax

    docker run ... -v $PWD/my_vol:/opt/my_vol ...

    The MountPoint you show in the docker volume inspect command is an internal implementation detail. On non-Linux hosts (with Linux containers) it’s hidden inside a virtual machine; even on native-Linux hosts directly accessing any of the filesystem content in /var/lib/docker is not a best practice.

    The most visible impact of putting VOLUME in your Dockerfile is that it will prevent any following RUN commands from modifying that directory tree. You don’t need it to mount content into the container and I’d highly recommend just deleting it (unless you’re 100% clear on what it does and why you want it).

    (Remember that anyone who has your image can trivially extract the ssh host keys and start their own ssh daemon impersonating your server. There are several significant security issues trying to run an sshd in Docker, including this, and I’d suggest avoiding it if at all possible.)

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