8.2.2.4 VOLUME Side Effects
A focused guide to VOLUME Side Effects, connecting core concepts with practical Docker and container operations.
VOLUME side effects refer to the consequences of an image's VOLUME instruction extending beyond the immediate build, affecting how every subsequently created container automatically receives an anonymous volume for the declared path, and how this can interact unexpectedly with later Dockerfile instructions or derived images.
The Automatic Anonymous Volume Side Effect
Every container created from an image with a VOLUME declaration automatically receives an anonymous volume for that path, whether or not the user explicitly requested one.
VOLUME /data
docker run -d myapp:1.0
This side effect occurs automatically, regardless of whether the user intended to use a volume at all for this particular run.
Side Effects on Later Build Instructions
An instruction in the Dockerfile after a VOLUME declaration attempting to write to that same path may not behave as expected, since the declared path is treated as an external mount point from that point forward.
VOLUME /data
RUN echo "test" > /data/file.txt
This instruction's effect is not reliably preserved in the resulting image, an easily overlooked side effect of the earlier VOLUME declaration.
Side Effects Inherited by Derived Images
An image built on top of a base image that declares a VOLUME inherits that same declaration, meaning every container created from the derived image also automatically receives this anonymous volume, even if the derived image's own Dockerfile never mentions VOLUME at all.
FROM postgres:16
A custom image built this way inherits the base image's VOLUME /var/lib/postgresql/data declaration automatically.
Why Awareness of These Side Effects Matters
Recognizing that VOLUME has consequences extending beyond simply documenting a path — automatic volume creation for every derived container, and interaction with later build instructions — helps avoid both unexpected behavior in a Dockerfile's later instructions and unexpected anonymous volume accumulation in containers created from images inheriting this declaration unknowingly.