How to mount volume from container to host in 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

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.