THRIFT-5349 Add net5.0 as supported platform
Client: netstd
Patch: Jens Geyer

This closes #2328
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index 531718c..a3c0b8e 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -126,7 +126,7 @@
 
 RUN apt-get install -y --no-install-recommends \
 `# dotnet core dependencies` \
-      dotnet-sdk-3.1
+      dotnet-sdk-5.0
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \