Mount seeded volume on linux host

I’d like to create a Database image and seed it with some initial data. This seems to work fine, since I’m able to create a container with a volume managed by docker. However, when I try to mount the volume to a directory on my linux host machine, it is created empty instead of seeded.

After several hours of trying different configurations, I narrowed down the problem to it’s core: the content of the folder in the container associated with the volume on the host machine is overwritten upon creation.

Below a simple Dockerfile that creates a folder containing a single file. When the container is started it prints out the content of the mounted folder.

FROM ubuntu:18.04

RUN mkdir /opt/test
RUN touch /opt/test/myFile.txt

VOLUME /opt/test

CMD ["ls", "/opt/test"]

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FROM ubuntu:18.04​RUN mkdir /opt/testRUN touch /opt/test/myFile.txt​VOLUME /opt/test​CMD ["ls", "/opt/test"]​

I’m building the image with: docker build -t test .

Volume managed by docker

$ docker run -v volume-test:/opt/test --name test test
myFile.txt

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$ docker run -v volume-test:/opt/test --name test testmyFile.txt​

Here I’m getting the expected output. With the volume mounted in the space managed by docker. So the ouput of docker volume inspect volume-test is:

{
  "CreatedAt": "2020-05-13T10:09:29+02:00",
  "Driver": "local",
  "Labels": null,
  "Mountpoint": "/var/snap/docker/common/var-lib-docker/volumes/volume-test/_data",
  "Name": "volume-test",
  "Options": null,
  "Scope": "local"
}

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{  "CreatedAt": "2020-05-13T10:09:29+02:00",  "Driver": "local",  "Labels": null,  "Mountpoint": "/var/snap/docker/common/var-lib-docker/volumes/volume-test/_data",  "Name": "volume-test",  "Options": null,  "Scope": "local"}​

Volume mounted on host machine

$ docker run -v $(pwd)/volume:/opt/test --name test test

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$ docker run -v $(pwd)/volume:/opt/test --name test test​

Where nothing is returned since the folder is empty… However, I can see that the volume is created and it’s owned by the user root, even though I’m executing the docker run command as another user.

drwxr-xr-x 2 root    root    4096 May 13 10:11 volume

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drwxr-xr-x 2 root    root    4096 May 13 10:11 volume​

As a last test, I tried to see what happens when I create the folder for the volume beforehand and add some content to it (in my case a file called anotherFile.txt). When I’m running now the container, I’m getting the following result:

$ docker run -v $(pwd)/volume:/opt/test --name test test
anotherFile.txt

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$ docker run -v $(pwd)/volume:/opt/test --name test testanotherFile.txt​

Which let’s me come to the conclusion, that the content in the folder of the container is overwritten by the content of the folder on the host machine.

I can verify as well with docker inspect -f '{{ .Mounts }}' test, that the volume is mounted at the right place:

[{bind  /pathWhere/pwd/pointedTo/volume /opt/test   true rprivate}]

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[{bind  /pathWhere/pwd/pointedTo/volume /opt/test   true rprivate}]​

Now my question: Is there a way to have the same behavior for volumes on the host machine as for the volumes managed by docker, where the content of the /opt/test folder in the container is copied into the folder on the host defined as volume?

Sidenote: this seems to be the case when using docker on windows and having the option Shared Folders enabled…

Furthermore, it seems as a similar question was already asked here but no answer was found. I decided to make a separate post since I think this is the most generic example to describe this issue.

Answer

Desired situation

Data from within the docker image is placed in a specified path on the host.

Your current situation

When creating the image, data is put into /opt/test
When starting the container, you mount the volume on /opt/test

Problem

Because you mount on the same path as where you have put your data, your data gets overwritten.

Solution

Create a file within image during docker-build, for example touch /opt/test/data/myFile.txt,
Use a different path to mount your volume, so that the data is not overwritten, for example /opt/test/mount
Use ‘CMD’ to copy the files to the volume, like so: CMD [“cp”, “-n” “/opt/test/data/*”, “/opt/test/mount/”]

Consulted sources

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Answer