docker image - mounted volume is empty-CodePudding

I am building my first docker image. I am a beginner

It is a simple python http server. This is my DockerFile

FROM python:3.8.0-slim
WORKDIR /src
COPY src/ .
CMD [ "python", "-m", "http.server", "--cgi", "8000"]

I have a config folder in /src with some config files. I named the image "my-server"

I create a container with

docker run -d \
    --name "my-server" \
    -p 8000:8000 \
    -v /dockerdata/appdata/my-server/config/:/src/config \
--restart unless-stopped \
my-server

the issue is /dockerdata/appdata/my-server/config/ is empty on my host. I see this done on all docker images on dockerhub I use and the mounted volumes are not empty for these images. How do they do it?

CodePudding user response：

Their startup sequence explicitly copies source files into the volume, or otherwise creates them. A Docker mount always replaces the content in the image with the content of whatever's being mounted; there is no way to mount the container content to the host.

(The one exception to this "always" is, if you're using native Docker, and you're mounting a named volume, and the named volume is empty, then content from the image is copied into the volume first; but the content is never ever updated, it only works for named volumes and not other kinds of mounts, and it doesn't work on other environments like Kubernetes. I would not rely on this approach.)

If the configuration is a single file, this isn't a huge imposition. You probably already need to distribute artifacts like a docker-compose.yml file separately from the image, so distributing a default configuration isn't much more. If defaults are compiled into your application and an empty configuration is valid, this also simplifies things. Another helpful approach could be to have a search path for configuration files, and read both a "user" and "system" configuration.

If you do need to copy files out to a host directory or other mount point, I would generally do this with an entrypoint wrapper script. You will need to keep a copy of the configuration in the image somewhere that's not the actual config directory so that you can copy it when it doesn't exist. The script can be fairly straightforward:

#!/bin/sh

# Copy the default configuration if it doesn't exist
if [ ! -f config/config.yml ]; then
  cp default-config/config.yml config
fi

# Run the main container command
exec "$@"

You may need to do some shuffling in your Dockerfile; the important thing is to make this script be the ENTRYPOINT but leave the CMD unchanged.

# Save the "normal" config away; the entrypoint script will create
# the "real" config if one isn't mounted
RUN mv config default-config \
 && mkdir config

# Launch the server via the entrypoint wrapper
ENTRYPOINT ["./entrypoint.sh"] # must be JSON array syntax
CMD ["python", "-m", "http.server", "--cgi", "8000"] # unchanged

CodePudding user response：

This is expected, bind mount to a container directory will result to the content in the directory to be obscured. If you mount to a named volume, the directory’s contents are copied into the volume.

docker run -d \
  --name "my-server" \
  -p 8000:8000 \
  -v myvol:/src/config \
--restart unless-stopped \
my-server

Now if you run docker run -it -v myvol:/config --rm busybox ls /config you will see the copied content.