Docker Volumes#

Docker provides three mechanisms for persisting or sharing data between a container and the outside world 1:

This article focuses on the volume mechanism and the VOLUME Dockerfile instruction.

The VOLUME Instruction#

VOLUME ["/path"] declares a mount point inside an image and marks it as externally managed 2. At container start, Docker automatically creates an anonymous volume and mounts it at the declared path unless an explicit -v is provided at docker run time.

Two syntax forms are accepted:

VOLUME ["/data"] # JSON array form (preferred)
VOLUME /data # shell form

The host directory cannot be specified inside a Dockerfile — it is resolved at container run-time to preserve image portability 3.

See the anonymous-volume example Dockerfile for the minimal pattern (note: RUN after VOLUME here writes to the working directory, not to /data, so the divergence described below does not apply).

VOLUME + RUN Ordering: The Critical Divergence#

The placement of VOLUME relative to RUN instructions is a common source of confusion, and the two Docker build backends behave differently 4:

Legacy builder: any changes made to the declared volume directory by subsequent RUN instructions are discarded.
BuildKit: changes made after VOLUME are retained in the image layer.

The course's TP10 Dockerfile illustrates this exact scenario 5:

1. Files are added to /mon-dossier before VOLUME — fichier1.txt, fichier2.txt
2. VOLUME ["/mon-dossier"] is declared
3. A third file is added after VOLUME — fichier3.txt

When you run a container from this image with BuildKit (the default since Docker 23.0), all three files are visible 6:

fichier1.txt fichier2.txt fichier3.txt

With the legacy builder, fichier3.txt would be missing because the post-VOLUME RUN write is silently dropped.

Practical rule: place all RUN instructions that seed volume content before the VOLUME declaration to behave consistently across both builders.

Anonymous vs. Named Volumes#

Anonymous volumes are created automatically when VOLUME is used in a Dockerfile or when -v /container/path (no name) is passed to docker run. They have a random ID and are not easily reused across containers 7.

Named volumes are created explicitly and can be shared between containers 8:

docker volume create my-data
docker run -v my-data:/data busybox

Key volume management commands:

docker volume ls
docker volume inspect my-data
docker volume rm my-data

Course Exercises (TP8–TP10)#

Primary Sources#

• Course chapter — content/chapitres/volumes.adoc — full slides and exercise solutions
• TP10 Dockerfile — content/code-samples/volumes/tp10/Dockerfile — the canonical VOLUME/RUN ordering example
• Anonymous volume Dockerfile — content/code-samples/volumes/anonymes/Dockerfile
• Docker official reference — VOLUME instruction docs 2