[FEATURE] - Add support for Docker secrets.

Relevant discussion:  https://github.com/bitbound/ControlR/discussions/105

The first thing I want to try is creating a [custom configuration provider](https://learn.microsoft.com/en-us/dotnet/core/extensions/custom-configuration-provider) for Docker secrets, so they can be easily added to the configuration builder in the same fashion as JSON files and environment variables.