Intended use of appProject in vscode docker-build task - docker

Question

Some context: I'm learning to use docker. I have a manually written Dockerfile and a docker-compose.yml file but when I stand up the container and the netcoreapp3.1 Kestrel web app contained therein, there's a problem loading the SSL certificate. The cert file isn't visible to Kestrel for reasons yet unknown to me.
In the process of fathoming this, I discovered VS Code has support for docker-build tasks. I've excerpted the interesting portion of the docs below.
The part that confuses me is the mandatory appProject property. My Dockerfile pulls source from git; several related projects. Then it compiles it and does various other things. Why do I need to specify an app project in the task when that sort of thing is defined by the Dockerfile? I strongly suspect I have failed to understand how these tasks are intended to be used — something the docs don't really address.
Could someone please explain the intended mode of use and the relevance of these settings? Preferably with examples.
Docker build task
The docker-build task builds Docker images using the Docker command line (CLI). The task can be used by itself, or as part of a chain of tasks to run and/or debug an application within a Docker container.
The most important configuration settings for the docker-build task are dockerBuild and platform:
The dockerBuild object specifies parameters for the Docker build command. Values specified by this object are applied directly to Docker build CLI invocation.
The platform property is a hint that changes how the docker-build task determines Docker build defaults.
Platform support#
While the docker-build task in tasks.json can be used to build any Docker image, the extension has explicit support (and simplified configuration) for Node.js, Python, and .NET Core
.NET Core (docker-build)#
Minimal configuration using defaults
When you build a .NET Core-based Docker image, you can omit the platform property and just set the netCore object (platform is implicitly set to netcore when netCore object is present). Note that appProject is a required property:
{
"version": "2.0.0",
"tasks": [
{
"label": "Build Node Image",
"type": "docker-build",
"netCore": {
"appProject": "${workspaceFolder}/project.csproj"
}
}
]
}
Platform defaults
For .NET Core-based images, the docker-build task infers the following options:
Property
Inferred Value
dockerBuild.context
The root workspace folder.
dockerBuild.dockerfile
The file Dockerfile in the root workspace folder.
dockerBuild.tag
The base name of the root workspace folder.