How to mount host volumes into docker containers in Dockerfile during build
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, ie, 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:
# BUILD-USING: docker build -t data .
# RUN-USING: docker run -name DATA data
FROM busybox
VOLUME ["/var/volume1", "/var/volume2"]
CMD ["/usr/bin/true"]
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
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.
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:
And more. These are the most critical issues that were blocking our adoption of Docker at Grammarly.
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.
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