dotnet template should include a volume mount for nuget package caching

Each time the container starts, all nuget packages need to be restored. The package and http caches should be persisted in volume mounts.

```json
"mounts": [
"source=NuGetLocal,target=/home/vscode/.local,type=volume",
"source=NuGet,target=/home/vscode/.nuget,type=volume"
],
"postCreateCommand": "chown vscode /home/vscode/.nuget /home/vscode/.local"
```

I don't totally understand the rhyme or reason for these two folders used by NuGet. They both contain configuration and caches, but of different kinds.

I also don't fully grasp how the mount system is supposed to work because the mounted volumes are owned by root instead of vscode by default, so I have to run chown as a postBuild script. At least [this is in the documentation](https://code.visualstudio.com/remote/advancedcontainers/improve-performance#_use-a-targeted-named-volume).