Docker ADD vs VOLUME

Question

I am learning Docker and I have doubts about when and where to use ADD and VOLUME. Here is what I think both of these do:

ADD

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Answer 1

ADD

The fundamental difference between these two is that ADD makes whatever you're adding, be it a folder or just a file actually part of your image. Anyone who uses the image you've built afterwards will have access to whatever you ADD. This is true even if you afterwards remove it because Docker works in layers and the ADD layer will still exist as part of the image. To be clear, you only ADD something at build time and cannot ever ADD at run-time.

A few examples of cases where you'd want to use ADD:

• You have some requirements in a requirements.txt file that you want to reference and install in your Dockerfile. You can then do: ADD ./requirements.txt /requirements.txt followed by RUN pip install -r /requirements.txt

• You want to use your app code as context in your Dockerfile, for example, if you want to set your app directory as the working dir in your image and to have the default command in a container run from your image actually run your app, you can do:

  ADD ./ /usr/local/git/my_app

  WORKDIR /usr/local/git/my_app

  CMD python ./main.py

VOLUME

Volume, on the other hand, just lets a container run from your image have access to some path on whatever local machine the container is being run on. You cannot use files from your VOLUME directory in your Dockerfile. Anything in your volume directory will not be accessible at build-time but will be accessible at run-time.

A few examples of cases where you'd want to use VOLUME:

• The app being run in your container makes logs in /var/log/my_app. You want those logs to be accessible on the host machine and not to be deleted when the container is removed. You can do this by creating a mount point at /var/log/my_app by adding VOLUME /var/log/my_app to your Dockerfile and then running your container with docker run -v /host/log/dir/my_app:/var/log/my_app some_repo/some_image:some_tag
• You have some local settings files you want the app in the container to have access to. Perhaps those settings files are different on your local machine vs dev vs production. Especially so if those settings files are secret, in which case you definitely do not want them in your image. A good strategy in that case is to add VOLUME /etc/settings/my_app_settings to your Dockerfile, run your container with docker run -v /host/settings/dir:/etc/settings/my_app_settings some_repo/some_image:some_tag, and make sure the /host/settings/dir exists in all environments you expect your app to be run.