Fix Dockerfile again

Dockerfile

@@ -7,6 +7,9 @@ ARG SQUIDEX__VERSION=4.0.0
 
 WORKDIR /src
 
+# Copy nuget project files.
+COPY backend/*.sln ./
+
 # Copy the main source project files
 COPY backend/src/*/*.csproj ./
 RUN for file in $(ls *.csproj); do mkdir -p src/${file%.*}/ && mv $file src/${file%.*}/; done