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问题:
Original question: How to use the VOLUME instruction in Dockerfile?
Revised: update from the answer below, so the actual question I want to solve is -- how to mount host volumes into docker containers in Dockerfile during build, i.e., having the docker run -v /export:/export capability during docker build.
Latest Update: There is a solution now. Although it is not strictly Docker, but "it solves all the weak points of Dockerfile", so I think it is the perfect answer. Check my latest answer for details.
Update: So the answer is "Not possible". I can accept it as an answer as I know the issue has been extensively discussed at https://github.com/docker/docker/issues/3156. I can understand that portability is a paramount issue for docker developer; but as a docker user, I have to say I'm very disappointed about this missing feature. Let me close my argument with a quote from aforementioned discussion: "I would like to use Gentoo as a base image but definitely don't want > 1GB of Portage tree data to be in any of the layers once the image has been built. You could have some nice a compact containers if it wasn't for the gigantic portage tree having to appear in the image during the install." Yes, I can use wget or curl to download whatever I need, but the fact that merely a portability consideration is now forcing me to download > 1GB of Portage tree each time I build a Gentoo base image is neither efficient nor user friendly. Further more, the package repository WILL ALWAYS be under /usr/portage, thus ALWAYS PORTABLE under Gentoo. Again, I respect the decision, but please allow me expressing my disappointment as well in the mean time. Thanks.
Original question in details:
From
Share Directories via Volumes
http://docker.readthedocs.org/en/v0.7.3/use/working_with_volumes/
it says that Data volumes feature "have been available since version 1 of the Docker Remote API". My docker is of version 1.2.0, but I found the example given in above article not working:
What's the proper way in Dockerfile to mount host-mounted volumes into docker containers, via the VOLUME command?
$ apt-cache policy lxc-docker lxc-docker: Installed: 1.2.0 Candidate: 1.2.0 Version table: *** 1.2.0 0 500 https://get.docker.io/ubuntu/ docker/main amd64 Packages 100 /var/lib/dpkg/status $ cat Dockerfile FROM debian:sid VOLUME ["/export"] RUN ls -l /export CMD ls -l /export $ docker build -t data . Sending build context to Docker daemon 2.56 kB Sending build context to Docker daemon Step 0 : FROM debian:sid ---> 77e97a48ce6a Step 1 : VOLUME ["/export"] ---> Using cache ---> 59b69b65a074 Step 2 : RUN ls -l /export ---> Running in df43c78d74be total 0 ---> 9d29a6eb263f Removing intermediate container df43c78d74be Step 3 : CMD ls -l /export ---> Running in 8e4916d3e390 ---> d6e7e1c52551 Removing intermediate container 8e4916d3e390 Successfully built d6e7e1c52551 $ docker run data total 0 $ ls -l /export | wc 20 162 1131 $ docker -v Docker version 1.2.0, build fa7b24f
回答1:
It is not possible to use the VOLUME instruction to tell docker what to mount. That would seriously break portability. This instruction tells docker that content in those directories does not go in images and can be accessed from other containers using the --volumes-from command line parameter. You have to run the container using -v /path/on/host:/path/in/container to access directories from the host.
Mounting host volumes during build is not possible. There is no privileged build and mounting the host would also seriously degrade portability. You might want to try using wget or curl to download whatever you need for the build and put it in place.
回答2:
UPDATE: Somebody just won't take no as the answer, and I like it, very much, especially to this particular question.
GOOD NEWS, There is a way now --
The solution is Rocker: https://github.com/grammarly/rocker
John Yani said, "IMO, it solves all the weak points of Dockerfile, making it suitable for development."
Rocker
https://github.com/grammarly/rocker
By introducing new commands, Rocker aims to solve the following use cases, which are painful with plain Docker:
- Mount reusable volumes on build stage, so dependency management tools may use cache between builds.
- Share ssh keys with build (for pulling private repos, etc.), while not leaving them in the resulting image.
- Build and run application in different images, be able to easily pass an artifact from one image to another, ideally have this logic in a single Dockerfile.
- Tag/Push images right from Dockerfiles.
- Pass variables from shell build command so they can be substituted to a Dockerfile.
And more. These are the most critical issues that were blocking our adoption of Docker at Grammarly.
回答3:
There is a way to mount a volume during a build, but it doesn't involve Dockerfiles.
The technique would be to create a container from whatever base you wanted to use (mounting your volume(s) in the container with the -v option), run a shell script to do your image building work, then commit the container as an image when done.
Not only will this leave out the excess files you don't want (this is good for secure files as well, like SSH files), it also creates a single image. It has downsides: the commit command doesn't support all of the Dockerfile instructions, and it doesn't let you pick up when you left off if you need to edit your build script.
回答4:
As you run the container, a directory on your host is created and mounted into the container. You can find out what directory this is with
$ docker inspect --format "{{ .Volumes }}" map[/export:/var/lib/docker/vfs/dir/]
If you want to mount a directory from your host inside your container, you have to use the -v parameter and specify the directory. In your case this would be:
docker run -v /export:/export data
SO you would use the hosts folder inside your container.
回答5:
I think you can do what you want to do by running the build via a docker command which itself is run inside a docker container. See Docker can now run within Docker | Docker Blog. A technique like this, but which actually accessed the outer docker from with a container, was used, e.g., while exploring how to Create the smallest possible Docker container | Xebia Blog.
Another relevant article is Optimizing Docker Images | CenturyLink Labs, which explains that if you do end up downloading stuff during a build, you can avoid having space wasted by it in the final image by downloading, building and deleting the download all in one RUN step.
