How to change default volume mount in VS Code Remote Container?

How to change the default volume mount for the folder opened in the container? I have tried in my Dockerfile:

RUN mkdir /root/myproject
WORKDIR /root/myproject

As well as my .devcontainer.json:

{
    "name": "My Project",
    "dockerFile": "Dockerfile",
    "workspaceFolder": "/root/myproject",
}

But the folder is still being mounted on /workspaces/myproject within the container.