chore(Dockerfile): bump dotnet 8

[bruno-garcia]

Trying to publish .NET SDK version 4.0.0 is failing due to missing .NET 8 SDK on the craft target.

dotnet: Install the [8.0.100-rc.2.23502.2] .NET SDK or update [/github/workspace/__repo__/global.json] to match an installed SDK.
dotnet: 
dotnet: Learn about SDK resolution:
dotnet: https://aka.ms/dotnet/sdk-not-found
dotnet: 

  STDOUT: dotnet: 7.0.402 [/usr/share/dotnet/sdk]
  dotnet:

  STDERR:dotnet: The command could not be loaded, possibly because:
  dotnet:   * You intended to execute a .NET application:
  dotnet:       The application '--version' does not exist.
  dotnet:   * You intended to execute a .NET SDK command:
  dotnet:       A compatible .NET SDK was not found.
  dotnet:
  dotnet: Requested SDK version: 8.0.100-rc.2.23502.2
  dotnet: global.json file: global.json
  dotnet:
  dotnet: Installed SDKs:
  dotnet:
  dotnet: Install the [8.0.100-rc.2.23502.2] .NET SDK or update [global.json] to match an installed SDK.
  dotnet:
  dotnet: Learn about SDK resolution:
  dotnet: https://aka.ms/dotnet/sdk-not-found
  dotnet:

We should be able to publish with the current .NET SDK version, so it's a separate thing to fix. Not sure how we're getting a global.json with net8.0-rc2 in there. But anyway once .NET 8 is out we'll want to bump this (next Monday).

[bruno-garcia]

exit code: 100

We need to wait until .NET 8 SDK is officially out

cc @bitsandfoxes