Understanding docker-compose of VS2017 webform application - asp.net

Question

I'm creating a Webform application that I try to containerize.
I use VS2017, create the default webform app, and add docker support.
If I F5 the application, it's working (hopefully)
The "add docker support" adds many files :
1) a Dockerfile, with
FROM microsoft/aspnet:4.6.2
ARG source
WORKDIR /inetpub/wwwroot
COPY ${source:-obj/Docker/publish} .
I understand it's based on the aspnet:4.6.2 image and that it copy the files from obj/docker/publish to put them in the container.
My problem is that I have no files published on this directory. Of course if I docker build, I have an error (it can't find files in obj/docker/publish). Remember, with F5 it's working.
But there's additionnal files.
2) Docker-compose.yml :
version: '3'
services:
legacywebapp:
image: legacywebapp
build:
context: .\LegacyWebApp
dockerfile: Dockerfile
If I understand well, it creates a legacywebapp services based on the dockerfile. The one that is not working, and still no "obj/docker/publish"
3) docker-compose.override.yml with :
version: '3'
services:
legacywebapp:
ports:
- "80"
networks:
default:
external:
name: nat
Ok, I assume it exposes the port 80 (why not in the first file ? but anyway, for modularity it's better I suppose).
If I try
docker-compose -f .\docker-compose.yml -f .\docker-compose.override.yml build
of course I have an error :
Step 4/4 : COPY ${source:-obj/Docker/publish} .
ERROR: Service 'legacywebapp' failed to build: COPY failed: GetFileAttributesEx \\?\C:\windows\TEMP\docker-builder798755009\obj\Docker\publish: The system cannot find the file specified.
Why is it working when I F5 in visual studio ? Let's look in the build output of VS, I can see the command line :
docker-compose -f "C:\Users\...\docker-compose.yml" -f "C:\Users\...\docker-compose.override.yml" -f "C:\Users\...\obj\Docker\docker-compose.vs.debug.g.yml" -p dockercompose15539612722162891187 up -d --build
So, there's another file, I suppose for debugging :
4) docker-compose.vs.debug.g.yml
version: '3'
services:
legacywebapp:
image: legacywebapp:dev
build:
args:
source: obj/Docker/empty/
volumes:
- C:\Users\...\LegacyWebApp:C:\inetpub\wwwroot
- C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Remote Debugger:C:\remote_debugger:ro
labels:
com.microsoft.visualstudio.debuggee.program: "C:\\app\\bin/LegacyWebApp.dll"
com.microsoft.visualstudio.debuggee.workingdirectory: "C:\\app"
And again, no obj/docker/publish. What is the obj/Docker/empty/ ?
How can I build correctly the image ? For example if I want to publish it in a private repository...