add --build-output-folder

[Patrick Stöckle (pstoeckle)]

Closes #1305

Co-authored-by: GitHub Copilot/GPT-5.6 Sol

[Patrick Stöckle (pstoeckle)]

In the build output folder, one then has a build-output.json like this

{
  "dockerfile": "/path/to/cli/build-output/Dockerfile",
  "context": "/path/to/repo/.devcontainer/build",
  "target": "dev_containers_target_stage",
  "buildArgs": {
    "_DEV_CONTAINERS_BASE_IMAGE": "dev_container_auto_added_stage_label",
    "_DEV_CONTAINERS_IMAGE_USER": "root",
    "_DEV_CONTAINERS_FEATURE_CONTENT_SOURCE": "dev_container_feature_content_temp"
  },
  "buildContexts": {
    "dev_containers_feature_content_source": "/path/to/cli/build-output"
  },
  "securityOpts": [],
  "imageNames": [
    "vsc-repo-25dfb64b7b2a1cbd22862334657ed284ed34cf8dbeca3d1275ee314d790f36f1"
  ]
}

One can then use, e.g., jq and the preferred container to build the container image based on the build folder.

docker build \
	--build-context dev_containers_feature_content_source=$(jq -r '.buildContexts.dev_containers_feature_content_source' build-output.json) \
	--build-arg _DEV_CONTAINERS_BASE_IMAGE=$(jq -r '.buildArgs._DEV_CONTAINERS_BASE_IMAGE' build-output.json) \
	.