Publish reference implementation

[chrmarti]

We want to extract a reference implementation for the dev container spec from our existing implementation. The current proposal is to make a CLI available that can take a devcontainer.json and create and configure a dev container from it.

Splitting it into 2 stages at first:

devcontainer up --workspace-folder .
devcontainer run-user-commands --workspace-folder .

up could be split into build and run to make building the image available separately.
run-user-commands runs "postCreateCommand" and other user-provided commands.

Products might choose to add to the container's setup between the two commands (e.g., Remote-Containers starts the ssh-agent forwarding among other things).

/cc @joshspicer @Chuxel @2percentsilk @bamurtaugh

[bamurtaugh]

Thanks for opening this!

up could be split into build and run to make building the image available separately.

I like this as it feels like it allows for easier iteration on the dev container.

[jkeech]

up could be split into build and run to make building the image available separately.

👍 This would be great to do. This would also allow us to prebuild devcontainer definitions using the CLI to generate a docker image that can be used as the image. For instance, if we transform the codespaces-linux definition to be a base image + a lot of devcontainer "features", then we would need to build the image through the CLI since it's more than just a plain docker build. But we don't want to spin up a container in that case, we just want to have the image built so we can push it to a registry.