Handle missing image/dockerfile/dockerComposeFile with definitions picker

[chrmarti]

Ref microsoft/vscode#96925

[chrmarti]

@Chuxel FYI: The idea here is to copy from a definition parts of the devcontainer.json properties into the existing devcontainer.json and copy all other files (Dockerfile, docker-compose.yml) in full. I'll merge lists like extensions and objects like settings.