Docker is a great tool to make our development much easier by introducing immutable and portable environment. But I had trouble to use Docker VOLUME yesterday. So basically, this is my note to describe what the problem was and the way how to solve it.

Volumes in Docker is a mechanism which enables us to generate data into host machine directory. Volumes are managed mainly by docker daemon, on the other hand, bind mounts is created on the user managed directory on the host machine. Official documentation says volumes have several advantages:

  • Volumes are easier to back up or migrate than bind mounts.
  • Volumes can be more safely shared among multiple containers.
  • Volume drivers allow you to store volumes on remote hosts or cloud providers, to encrypt the contents of volumes, or to add other functionality.

docker volumes

But we need to pay attention to the writability when we use VOLUME directive in Dockerfile. That may cause unnecessary trouble if you understand correctly like me. Let’s say we are using a Dockerfile that looks like this:

FROM ubuntu:16.04

VOLUME /myvol
RUN echo 'Hello World' >> /myvol/hello

$ docker build -t lewuathe/myvol .
$ docker run -it lewuathe/myvol bash
# ls /myvol
total 8
drwxr-xr-x 2 root root 4096 Mar  8 00:44 ./
drwxr-xr-x 1 root root 4096 Mar  8 00:44 ../

You won’t be able to find the file hello. Why? The official documentation says that:

Changing the volume from within the Dockerfile: If any build steps change the data within the volume after it has been declared, those changes will be discarded.

So you cannot change the directory/file that is created by VOLUME from Dockerfile. Of course, you can change the content from the process in running container after it launches. That must be the limitation of VOLUME in Dockerfile.

This is one solution.

FROM ubuntu:16.04

RUN mkdir /myvol
RUN echo 'Hello World' >> /myvol/hello
VOLUME /myvol

You can write anything before creating volumes. Creating volumes at the last will enable you to do what you want. So please keep in mind that the changes done before the volume creation will be discarded.

Thanks.

Reference