fix: always write devcontainer.metadata label as JSON array

[fechu]

The getDevcontainerMetadataLabel function wrote a bare JSON object when there was only one metadata entry (e.g. docker-compose devcontainer with no features), but an array when there were multiple entries

• The spec states the label "can contain an array of json snippets"
• Tools like Zed (parsing code) or envbuilder that follow the spec and expect an array fail

The reading side (internalGetImageMetadata0) already handles both formats, but the writing side was inconsistent. In this PR we propose to always write an array of objects for simplicity. For backwards compatibility we leave the parsing, which supports both. Changing this makes the code follow the robustness principle by being strict what is returned (always an array).

Test instructions

Create the following project:

/tmp/zed-devcontainer-repro
└── .devcontainer
    ├── devcontainer.json
    ├── docker-compose.yml
    └── Dockerfile

devcontainer.json:

{
  "name": "repro",
  "dockerComposeFile": "docker-compose.yml",
  "service": "app",
  "remoteUser": "root"
}

docker-compose.yml:

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
    command: sleep infinity
    volumes:
      - ..:/workspace

Dockerfile

FROM ubuntu:24.04

With the latest release the metadata is:

> devcontainer up --workspace-folder /tmp/zed-devcontainer-repro
[...]
> docker inspect --format='{{index .Config.Labels "devcontainer.metadata"}}' \
    $(docker ps -q --filter "label=devcontainer.local_folder=/tmp/zed-devcontainer-repro")

{"remoteUser":"root"}

After this change:

[{"remoteUser":"root"}]

Fixes #1054

[fechu]

@microsoft-github-policy-service agree

[abdurriq]

Thank you for fixing this!