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I am pretty new to docker, so i might be doing something truly wrong

I need to share some files between docker containers, using a docker compose file

I have already created a volume like this

docker volume create shared

After that i can check the created volume

docker volume inspect shared
[
 
 "CreatedAt": "2019-03-08T14:54:57-05:00",
 "Driver": "local",
 "Labels": ,
 "Mountpoint": "/var/lib/docker/volumes/shared/_data",
 "Name": "shared",
 "Options": ,
 "Scope": "local"
 
]

My docker-compose.yaml file looks like this

version: '3.1'

services:
 service1:
 build:
 context: Service1
 dockerfile: Dockerfile
 restart: always
 container_name: server1-server
 volumes:
 - shared:/shared

 service2:
 build:
 context: Service2
 dockerfile: Dockerfile
 restart: always
 container_name: server2-server
 volumes:
 - shared:/shared

volumes:
 shared:
 external: true

And the Dockerfile looks like this (just for testing purposes)

FROM microsoft/dotnet:2.2-sdk AS build-env

RUN echo "test" > /shared/test.info

When i issue a docker-compose up command i get this error

/bin/sh: 1: cannot create /shared/test.info: Directory nonexistent

If i modify the Dockerfile to this

FROM microsoft/dotnet:2.2-sdk AS build-env

WORKDIR /app

COPY *.csproj ./
RUN cp *.csproj /shared/ 

I get this error

cp: cannot create regular file '/shared/': Not a directory

Any ideas how to achieve this ?

A Dockerfile contains instructions to create an image. After the image is built, the image can be run as a container.

A volume is attached when launching containers.

It thus makes no sense to use Dockerfile instructions to copy a file into a volume while building an image.

Volumes are generally used to share runtime data between containers, or to keep data after a container is stopped.