`devcontainer.json` support

`.devcontainer/devcontainer.json` has an [`image`](https://containers.dev/implementors/json_reference/#:~:text=Description-,image,-string) property. I want to pin it using `dockerfile-pin`.

```json
{
"name": "node",
"image": "mcr.microsoft.com/devcontainers/javascript-node:24",
"settings": {
"terminal.integrated.defaultProfile.linux": "bash"
},
"extensions": [
"dbaeumer.vscode-eslint",
"esbenp.prettier-vscode"
],
"postCreateCommand": "npm install"
}
```

The field is not required when `dockerComposeFile` or `build.dockerfile` is specified.

```json
{
"dockerComposeFile": "../compose.yaml",
"service": "app",
"workspaceFolder": "/src",
}
```

```json
{
"name": "go",
"build": {
"dockerfile": "../Dockerfile"
}
}
```

Start by reading the existing Dockerfile, docker-compose.yml, and GitHub Actions pinning entry points, then trace how an image reference is detected and rewritten. Add support for the image property in .devcontainer/devcontainer.json while leaving configurations using dockerComposeFile or build.dockerfile unaffected. Verify that the devcontainer image receives a digest pin.

Written by the indexing model from the issue text.