devcontainer.json shows Warning/Problem for extension entry using a filepath

• VSCode Version: 1.81.0, 1.82.0-insider
• Local OS Version: Windows 10
• Remote OS Version: N/A
• Remote Extension/Connection Type: Happens locally with no extensions installed. Even though this happens with no extensions installed, I logged it here because it's related to the devcontainer.json file format.

Steps to Reproduce:

1. Make a simple devcontainer.json file like this

{
    "name": "MyDevContainer",
    "image": "ubuntu:20.04",

    "customizations": {
		"vscode": {
            "extensions": [
                "ms-vscode.cpptools",
                "/extensions/redhat.java-0.82.0.vsix"
            ]
        }
    }
}

2. This shows a Problem with the "/extensions/redhat.java-0.82.0.vsix" line and the following text: Expected format: '${publisher}.${name}' or '${publisher}.${name}@${version}'. Example: 'ms-dotnettools.csharp'.
3. This does install the extension via the filesystem path.

Does this issue occur when you try this locally?: Yes
Does this issue occur when you try this locally and all extensions are disabled?: Yes

Additional Notes:

1. See this comment that documents the format: https://github.com/microsoft/vscode-dev-containers/issues/1572#issuecomment-1209977897
2. My Use Case:
   a. I'm using devcontainers for our development environment, and I want to ensure that one of our custom extensions is installed for developers automatically, like the extensions that are available on the marketplace (ms-vscode.cpptools).
   b. If there's an easier way to ensure that a custom extension is installed inside the container automatically, that would work for me too (and make this issue N/A for me)