Update Dockerfile to add package FsToolkit.ErrorHandling (#224)

Adding package FsToolkit.ErrorHandling to pre-installed packages for F# as discussed in this thread: https://forum.exercism.org/t/criteria-for-adding-nuget-packages-result-helper-library-would-be-nice/11164

Author: mattnewport

Dockerfile

@@ -21,6 +21,7 @@ RUN dotnet add package FSharp.Core -v 6.0.1
 RUN dotnet add package FSharp.Core -v 7.0.400
 RUN dotnet add package FSharp.Core -v 8.0.101
 RUN dotnet add package FParsec -v 1.1.1
+RUN dotnet add package FsToolkit.ErrorHandling -v 4.15.2
 
 WORKDIR /app