5

Say there is an image A described by the following Dockerfile:

FROM bash
RUN mkdir "/data" && echo "FOO" > "/data/test"
VOLUME "/data"

I want to specify an image B that inherites from A and modifies /data/test. I don't want to mount the volume, I want it to have some default data I specify in B:

FROM A
RUN echo "BAR" > "/data/test"

The thing is that the test file will maintain the content it had at the moment of VOLUME instruction in A Dockerfile. B image test file will contain FOO instead of BAR as I would expect.

The following Dockerfile demonstrates the behavior:

FROM bash
# overwriting volume file
RUN mkdir "/volume-data" && echo "FOO" > "/volume-data/test"
VOLUME "/volume-data"
RUN echo "BAR" > "/volume-data/test"
RUN cat "/volume-data/test" # prints "FOO"
# overwriting non-volume file
RUN mkdir "/regular-data" && echo "FOO" > "/regular-data/test"
RUN echo "BAR" > "/regular-data/test"
RUN cat "/regular-data/test" # prints "BAR"

Building the Dockerfile will print FOO and BAR.

Is it possible to modify file /data/test in B Dockerfile?

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3 Answers 3

Reset to default
2

It seems that this is intended behavior.

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.

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1 Comment

Even if the behavior is intended it could be useful if they gave a way to modify the data.
1

VOLUMEs are not part of your IMAGE. So what is the use case to seed data into it. When you push the image into another location, it is using an empty volume at start. The dockerfile behaviour does remind you of that.

So basically, if you want to keep the data along with the app code, you should not use VOLUMEs. If the volume declaration did exist in the parent image then you need to remove the volume before starting your own image build. (docker-copyedit).

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0

There are a few non-obvious ways to do this, and all of them have their obvious flaws.

Hijack the parent docker file

Perhaps the simplest, but least reusable way, is to simply use the parent Dockerfile and modify that. A quick Google of docker <image-name:version> source should find the github hosting the parent image Dockerfile. This is good for optimizing the final image, but destroyes the point of using layers.

Use an on start script

While a Dockerfile can't make further modifications to a volume, a running container can. Add a script to the image, and change the Entrypoint to call that (and have that script call the original entry point). This is what you will HAVE to do if you are using a singleton-type container, and need to partially 'reset' a volume on start up. Of course, since volumes are persisted outside the container, just remember that 1) your changes may already be made, and 2) Another container started at the same time may already be making those changes.

Since volumes are (virtually) forever, I just use one time setup scripts after starting the containers for the first time. That way I easily control when the default data is setup/reset. (You can use docker inspect <volume-name> to get the host location of the volume if you need to)

The common middle ground on this one seems to be to have a one-off image whose only purpose is to run once to do the volume configurations, and then clean it up.

Bind to a new volume

Copy the contents of the old volume to a new one, and configure everything to use the new volume instead.

And finally... reconsider if Docker is right for you

You probably already wasted more time on this than it was worth. (In my experience, the maintenance pain of Docker has always far outweighed the benefits. However, Docker is a tool, and with any tool, you need to sometimes take a moment to reflect if you are using it right, and if there are better tools for the job.)

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