如何在不使用Docker主机安装主机的情况下共享特定文件？

Question

I have a container from which I want to expose files to another container without using host mounting.

In my Dockerfile I declared the /etc/nginx/ directory as a volume after I copied the necessary files:

COPY ./src/frontend/config/nginx.conf /etc/nginx/nginx.conf
COPY ./src/frontend/config/sites-available/default /etc/nginx/sites-enabled/default
VOLUME ["/etc/nginx/"]

But because the whole directory acts as a volume the original contents of /etc/nginx/ is shadowed by the contents of my volume:

dk run --volumes-from other_container -it --rm nginx:1.12.1 /bin/bash
> ls /etc/nginx
> nginx.conf sites-enabled

This is something which I'd like to avoid. However it seems that Docker doesn't really support exposing an existing file as a volume.

VOLUME ["/etc/nginx/nginx.conf"]

If I use the volume as above I get the following error message:

cannot mount volume over existing file, file exists /var/lib/docker/overlay2/b557a0d1ef5026f581f7ee69321de854d8f77b935c0442458f26cada855e5bf2/merged/etc/nginx/nginx.conf

Is it possible to solve this problem without using host mounting?

Answer 1

When Docker volumes are mounted, they hide any previous content on the mountpoint.

For this reason, the volume has to be mounted in a separate directory, for instance /etc/nginx/extra/ or anywhere else. All other config files need to be adapted in order to make the includes from the right directory.