Variable expansion in secrets file

Hi,

I would like to express my sincere gratitude to the maintainers of this wonderful tool.

I have a question concerning the `--secrets-file` option. Currently, the file passed to `--secrets-file` is interpreted as a raw jsonc file. However, I would like to use some variable syntax, which is already used in devcontainer.json.

For example, I would like to use the following syntax in my secrets file:

```.json
{
"GITHUB_TOKEN": "${localEnv:GITHUB_TOKEN}"
}
```

instead of:

```.json
{
"GITHUB_TOKEN": "ghp********"
}
```

I would like to know the maintainer team's opinion on whether this idea is reasonable or not.

Thank you in advance.