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

This closes #2328
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index e554c53..20ee117 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -122,7 +122,7 @@
 
 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` \