Understanding volume instructions in Docker
In Docker, we can create separate images, containers, and volumes. Whenever we start a container, we can mount the volume.
If we can do it manually in Docker, we can automate it through a build process using a file called Dockerfile.
This article will discuss building, creating, and mounting volumes in Docker via a Dockerfile.
Understanding the VOLUME instruction in Docker
When using Dockerfile, we use several instructions to automatically build single or multiple containers. An example of these instructions is the VOLUME instruction.
In the Dockerfile, it should look like this:
FROM ubuntu
RUN mkdir /samplevol
VOLUME /samplevol
However, some confusion arises when we try to enter the value of the VOLUME instruction in the Dockerfile. When using the VOLUME instruction, we need to understand that this is not where we indicate the location of the Docker volume outside the container.
Instead, the VOLUME instruction is where we tell the target where inside the container we are going to mount our external Docker volume when the container is already running.
If we remember, when we run a container we can use the --volume or -v parameter to mount a specific volume. However, this parameter has other uses as well.
For example, using the --volume parameter we can specify where the Docker volume should be mounted.
Example command:
$ docker run --volume=/volumesd:/samplevol sushi_bake
In the above command, the /volumesd directory is the location of the unmounted Docker volume. The /samplevol directory is where we will mount the Docker volume in the container we create and run.
Additionally, the **/samplevol** directory is the correct value for the VOLUME instruction in the Dockerfile.
Alternatively, we can avoid confusion by not adding the VOLUME instruction in the Dockerfile. If we do not add a mount point in the VOLUME instruction, Docker assigns a default location for the volume to be mounted.
This location is usually under /var/lib/docker/volumes .
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