Update Dockerfile

Author: gregkalapos

docker/dotnet/Dockerfile

@@ -15,6 +15,9 @@ COPY . /src
 # install SDK version in global.json of elastic/apm-agent-dotnet
 # Needed when building branches that specify 3.1.100 SDK in global.json
 RUN curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin --install-dir ${DOTNET_ROOT} -version 3.1.100
+
+# SDK 5.x is also needed
+RUN curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin --install-dir ${DOTNET_ROOT} -version 5.0.100
 RUN ./run.sh
 
 # Stage 2: Run the TestAspNetCoreApp app