How to mount volume from container to host in Docker? - docker

Question

I have a question regarding the whole data volume process in Docker. Basically here are two Dockerfiles and their respective run commands:
Dockerfile 1 -
# Transmission over Debian
#
# Version 2.92
FROM debian:testing
RUN apt-get update \
&& apt-get -y install nano \
&& apt-get -y install transmission-daemon transmission-common transmission-cli \
&& mkdir -p /transmission/config /transmission/watch /transmission/download
ENTRYPOINT ["transmission-daemon", "--foreground"]
CMD ["--config-dir", "/transmission/config", "--watch-dir", "/transmission/watch", "--download-dir", "/transmission/download", "--allowed", "*", "--no-blocklist", "--no-auth", "--no-dht", "--no-lpd", "--encryption-preferred"]
Command 1 -
docker run --name transmission -d -p 9091:9091 -v C:\path\to\config:/transmission/config -v C:\path\to\watch:/transmission/watch -v C:\path\to\download:/transmission/download transmission
Dockerfile 2 -
# Nginx over Debian
#
# Version 1.10.3
FROM debian:testing
RUN apt-get update \
&& apt-get -y install nano \
&& apt-get -y install nginx
EXPOSE 80 443
CMD ["nginx", "-g", "daemon off;"]
Command 2 -
docker run --name nginx -d -p 80:80 -v C:\path\to\config:/etc/nginx -v C:\path\to\html:/var/www/html nginx
So, the weird thing is that the first dockerfile and command works as intended. Where the docker daemon mounts a directory from the container to the host. So, I am able to edit the configuration files as I please and they will be persisted to the container on a restart.
However, as for the second dockerfile and command it doesn't seem to be working. I know if you go to the Docker Volume documentation it says that volume mounts are only intended to go one-way, from host-to-container, but how come the Transmission container works as intended, while the Nginx container doesn't?
P:S - I'm running Microsoft Windows 10 Pro Build 14393 as my host and Version 17.03.0-ce-win1 (10300) Channel: beta as my Docker version.
Edit - Just to clarify. I'm trying to get the files from inside the Nginx container to the host. The first container (Transmission) works in that regard, by using a data volume. However, for the second container (Nginx), it doesn't want to copy the files in the mounted directory from inside the container to the host. Everything else is working though, it does successfully start.

Answer 1

The host volume will not copy data like a named volume will. However, you can create a named volume that performs a bind mount, which will then have the data initialization properties of any other named volume. The only prerequisite of a bind mount over a host volume is that the directory must exist in advance, docker will not create it for you like it does with a host volume. Here are three different examples of how to create a bind mount volume:
# create the volume in advance
$ docker volume create --driver local \
--opt type=none \
--opt device=/home/user/test \
--opt o=bind \
test_vol
# create on the fly with --mount
$ docker run -it --rm \
--mount type=volume,dst=/container/path,volume-driver=local,volume-opt=type=none,volume-opt=o=bind,volume-opt=device=/home/user/test \
foo
# inside a docker-compose file
...
volumes:
bind-test:
driver: local
driver_opts:
type: none
o: bind
device: /home/user/test
...
So in your example with a docker run command, you can use the mount syntax:
docker run --name nginx -d -p 80:80 \
--mount type=volume,dst=/etc/nginx,volume-driver=local,volume-opt=type=none,volume-opt=o=bind,volume-opt=device=/c/path/to/config \
--mount type=volume,dst=/var/www/html,volume-driver=local,volume-opt=type=none,volume-opt=o=bind,volume-opt=device=/c/path/to/html \
nginx
The only part that may need adjusting is the windows path names inside the Linux VM that docker runs in HyperV.

Answer 2

Host volumes don't copy data from the container > host. Host volumes mount over the top of what's in the container/image, so they effectively replace what's in the container with what's on the host.
A standard or "named" volume will copy the existing data from the container image into a new volume. These volumes are created by launching a container with the VOLUME command in it's Dockerfile or by the docker command
docker run -v myvolume:/var/whatever myimage
By default this is data stored in a "local" volume, "local" being on the Docker host. In your case that is on the VM running Docker rather than your Windows host so might not be easily accessible to you.
You could be mistaking transmission auto generating files in a blank directory for a copy?
If you really need the keep the VM Host > container mappings then you might have to copy the data manually:
docker create --name nginxcopy nginx
docker cp nginxcopy:/etc/nginx C:\path\to\config
docker cp nginxcopy:/var/www/html C:\path\to\html
docker rm nginxcopy
And then you can map the populated host directories into the container and they will have the default data the image came with.