Migrate off of Cottle templating engine

Cottle, our templating library, presents some usability issues with our Dockerfile and Readme templates:

1. Confusing syntax with no linter/syntax highlighting support
2. Errors are difficult to track down and it's sometimes unclear what the problem is
3. Managing whitespace is difficult

We should migrate to a different templating engine. The ideal templating engine would fulfill the following requirements:

1. Extensible, so that we can add our own custom functions/commands
2. Composable (Not strictly required, we currently shoehorn this into Cottle via its extensibility)
3. General-purpose enough to work for both Dockerfiles and text (Markdown)
4. Syntax highlighting and clear errors
5. Good support

Some options immediately come to mind:
1. [Razor](https://learn.microsoft.com/en-us/aspnet/core/mvc/views/razor?view=aspnetcore-9.0) - built in to .NET, but requires ASP.NET Core libraries
2. [T4](https://learn.microsoft.com/en-us/visualstudio/modeling/writing-a-t4-text-template?view=vs-2022)
3. [Fluid (Liquid)](https://github.com/sebastienros/fluid) - port of Liquid templating engine to .NET, maintained by .NET team member, popular on NuGet