--platform and --push flags force the use of a Dockerfile even if features are referenced

[Chuxel]

The --platform and --push flags are important enablers for pre-building images - particularly those that take advantage of Dev Container Features (devcontainers/spec#61).

Unfortunately, right now there's an error if only an image is referenced along with a set of features in devcontainer.json. Given just being able to just start from an image is one of the primary goals of Dev Container Features, this seems to be pretty clearly a bug.

This occurred in version 0.9.1 of the CLI.

Error: --platform or --push require dockerfilePath.
    at doBuild (/usr/local/share/npm-global/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:342:23)
    at async build (/usr/local/share/npm-global/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:233:20)
{"outcome":"error","message":"--platform or --push require dockerfilePath.","description":"--platform or --push require dockerfilePath."}

//cc @chrmarti @joshspicer @alexdima @bamurtaugh

[joshspicer]

This seems intentional - @juzuluag do you recall what scenario this was attempting to prevent?
https://github.com/devcontainers/cli/blob/main/src/spec-node/devContainersSpecCLI.ts#L396

[Chuxel]

Before features was implemented, this would make sense. Post-features, it doesn't - so I'm assuming this was just a good check that no longer applies.

[juzuluag]

This seems intentional - @juzuluag do you recall what scenario this was attempting to prevent?
https://github.com/devcontainers/cli/blob/main/src/spec-node/devContainersSpecCLI.ts#L396

Hi, @joshspicer. As far I recall, it was to prevent an error when there was no dockerfile file specified as part of .devcontainer/devcontainer.json. Doing unit test, it was failing using the feature test folders. Not sure whether Features merged first before this functionality merged.

[chrmarti]

I missed that case when we brought the multi-arch changes in on-top of then recent work on features. I see buildAndExtendDockerCompose in dockerCompose.ts handles the image case, so that could serve as a guide.