Docker run command has option to mount host directory into container
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.
And Dockerfile has VOLUME instruction
VOLUME ["/data"] - The VOLUME instruction will add one or more new volumes
to any container created from the image.
From what I see, there is no way to specify host-dir or rw/ro status when using Dockerfile.
Is there any other use of VOLUME in docker file other than wanting to share it with some other container?
Dockerfiles are meant to be portable and shared. The host-dir volume is something 100% host dependent and will break on any other machine, which is a little bit off the Docker idea.
Because of this, it is only possible to use portable instructions within a Dockerfile. If you need a host-dir volume, you need to specify it at run-time.
A common usage of VOLUME from Dockerfile is to store configuration or website sources so that it can be updated later by another container.
Related
I want to mount my host path (or docker volume) to the path in the image from dockerfile.
Dockerfile can copy the host data or directory to the data or directory in the image.
But I want to bind or mount not copy!
It is similar to "docker run -v" option but I wonder that it is done by creating the image from dockerfile.
Is any solution for this issue?
No, as far as I am aware, a host path cannot be mounted via the Dockerfile due to the portability of docker images and the different host architectures/directory layouts etc. See dockerfile volumes note 4.
Using VOLUME within the dockerfile will create a docker volume on the host at run-time of the container, but it cannot be specified to be a host directory. This answer explains the use of dockerfile VOLUME quite well. To use a host directory, you will need to do it at run time.
Docker run command has option to mount host directory into container
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.
And Dockerfile has VOLUME instruction
VOLUME ["/data"] - The VOLUME instruction will add one or more new volumes
to any container created from the image.
From what I see, there is no way to specify host-dir or rw/ro status when using Dockerfile.
Is there any other use of VOLUME in docker file other than wanting to share it with some other container?
Dockerfiles are meant to be portable and shared. The host-dir volume is something 100% host dependent and will break on any other machine, which is a little bit off the Docker idea.
Because of this, it is only possible to use portable instructions within a Dockerfile. If you need a host-dir volume, you need to specify it at run-time.
A common usage of VOLUME from Dockerfile is to store configuration or website sources so that it can be updated later by another container.
From https://hub.docker.com/r/gitlab/gitlab-runner/dockerfile:
VOLUME ["/etc/gitlab-runner", "/home/gitlab-runner"]
I have read Understanding "VOLUME" instruction in DockerFile and docker volume and VOLUME inside Dockerfile, however, I still cannot figure why does this image specify these two paths as VOLUME.
The VOLUME command in the Dockerfile exposes these directories as VOLUMES. Meaning it will auto-create volumes on docker run to persist the data inside the container(s).
Since containers are suppose to be stateless you want to persistent configuration and e.g. build-cache on the host.
I want to mount a folder of the host system to the container and it need to be defined in the Dockerfile so that user doesn't need to do it manually by passing the argument in the command line to run the container. How to achieve this ?
This simply cannot be done. Docker images are designed to be portable. Host mounts are host specific. Thus if you are able to specify a host mount at build time, it will make the image non-portable across machine that don't have this mount folder. Thus this is why this option is not available.
You can use docker compose to help the user not choose the mount folder. Take a look at How do I mount a host directory as a volume in docker compose
Dockerfile is for create images not containers.
You can not define a volume on a image. The volume must be defined on execution time when the container is created.
docker run -v /host-folder:/root/containerfolder -i -t imagename
Is there any difference between:
Mounting a host directory into a container path (the path is not exposed as a volume), e.g.:
docker run -v /host/directory:/container/directory my_image command_to_run
Dockerfile of my_image does not include VOLUME ["/container/directory"]
Mounting a host directory into a a container path exposed as a volume:
docker run -v /host/directory:/container/directory my_image command_to_run
Dockerfile of my_image includes VOLUME ["/container/directory"]
I know that volume data persists independent of the container life-cycle. However, since I want to work on my local data from within a container, does that make any difference if the mount-point inside the container is a volume?
There is no difference if you mount the path from the host into the container. The filesystem from the host will be mounted over top of that directory inside the container.
The difference between listing the volume and not listing it inside the image is the behavior of docker when you create an image without specifying a volume. When the volume is defined on the image, docker will create an "anonymous" volume you can see with docker volume ls as a long uuid string. These volumes are rarely useful, so I recommend against defining a volume in the image and instead only defining them on your docker run command or docker-compose.yml definition.
Downsides of defining a volume in the image include:
Later lines in the Dockerfile or in descendant Dockerfile's may not be able to change the contents at this location. Docker's behavior with this varies by scenario and version, so for predictability, once a volume is defined in an image, I consider that directory off limits.
Creation of anonymous volumes are difficult to use and are likely to clutter up the filesystem.
I posted a blog on this topic a while back if you're interested in more details.