Migrate off of Cottle templating engine

[lbussell]

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 - built in to .NET, but requires ASP.NET Core libraries
2. T4
3. Fluid (Liquid) - port of Liquid templating engine to .NET, maintained by .NET team member, popular on NuGet

[lbussell]

[Triage] Related: dotnet/dotnet-docker#6251