THRIFT-4990 Switch to .NET core 3.0
Client: netstsd
Patch: Jens Geyer

This closes #1909
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 8dc6497..714a80d 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -127,7 +127,7 @@
 
 RUN apt-get install -y --no-install-recommends \
 `# dotnet core dependencies` \
-      dotnet-sdk-2.1
+      dotnet-sdk-3.0
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \