Docker image doesn't support .NET 6

[Doug-Murphy]

🐛 Bug Report

The Docker image currently only uses .NET 5 SDK and therefore does not support C# projects using .NET 6.

Expected behavior

Running the nukeeper Docker image against a .NET 6 project with outdated NuGet packages successfully creates a PR with updates

Reproduction steps

1. Create a .NET 6 C# project
2. Add outdated NuGet package versions
3. Run the nukeeper Docker image against this project with valid configuration
4. Observe the following error message:

Updating 1 package
'Serilog' from 1.5.14 to 2.10.0 in 1 projects
Git checkout local branch 'main'
Failed on repo automaticnugetupdatetesting NuKeeperException : Command dotnet failed with exit code: 1
  Determining projects to restore...
/usr/share/dotnet/sdk/5.0.401/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [/tmp/NuKeeper/repo-49fddae4da5b4dd78af39413299d82de/AutomaticNuGetUpdateTestingLibrary1/AutomaticNuGetUpdateTestingLibrary1.csproj]

Configuration

Version: 0.35.0

Platform if applicable:

• 🛠️ NuKeeper CLI
• ✨ GitHub
• 🤖 AzureDevops
• 🏁 Bitbucket
• 🌎 Gitlab
• 📺 Gitea
• 🐳 Docker