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.

Start by tracing the --secrets-file entry point and its raw JSONC parsing, then compare it with the variable expansion already used in devcontainer.json. The issue does not define an implementation scope or acceptance criteria, so confirm the maintainers' desired behavior before estimating completion.

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