Add comment 'from URL' and 'do not edit' to built-in devcontainer.json

[chrmarti]

With microsoft/vscode-remote-release#3293 it has become very easy for new users to open a built-in devcontainer.json. For their info and to prevent them loosing changes we should include a comment at the top of every built-in config pointing the user to the source in this repository and advising them not to edit the file because the changes will be lost when the extension updates.

[Chuxel]

@chrmarti Just to clarify, are you talking about for repository-containers specifically? If so, that's a good call out. (For containers, there's already a pointer.)

[chrmarti]

Yes, those configurations in repository-containers. We don't have that many yet, but with the (delayed) automatic activation of Remote-Containers and the link for opening the devcontainer.json in the Reopen notification, it seems likely that we will see (especially new) users edit these built-in configurations and then loosing their changes when the extension updates.

Not sure how many of these we will see, but maybe we can at least include something minimal at the top of each for this milestone. Like:

// NOTE: Changes to this file and files in this folder will be lost on the next update of the Remote-Containers extension.

References to the documentation and the source of the config would make sense too.

[Chuxel]

@chrmarti Yeah I was thinking we could actually tell people that if they want to make modifications they can copy the contents to a local repository folder now that we have that.

[chrmarti]

I agree, we can also think about adding a command to help with that.

[chrmarti]

We could offer to copy a configuration to one of the configuration folders in the settings if there is one and also to the workspace folder through a command. I'd first wait and see if this is indeed something users request / need, but there are definitely options for improvement.

[chrmarti]

Current suggestion for the June milestone is to open the devcontainer.json in the browser on GitHub: microsoft/vscode-remote-release#3322. We that we don't need the comments for now.