THRIFT-4990 Upgrade to .NET Core 3.1 (LTS)
Client: netstd
Patch: Jens Geyer

This closes #1961
diff --git a/build/docker/README.md b/build/docker/README.md
index e33c4e0..8c8be22 100644
--- a/build/docker/README.md
+++ b/build/docker/README.md
@@ -177,7 +177,6 @@
 | d         | 2.075.1       | 2.087.0       |       |
 | dart      | 2.0.0         | 2.4.0         |       |
 | delphi    |               |               | Not in CI |
-| dotnet    | 3.0           | 3.0           |       |
 | erlang    | 18.3          | 22.0          |       |
 | go        | 1.10.8        | 1.12.6        |       |
 | haskell   | 7.10.3        | 8.0.2         |       |
@@ -185,6 +184,7 @@
 | java      | 1.8.0_191     | 11.0.3        |       |
 | js        |               |               | Unsure how to look for version info? |
 | lua       |               | 5.2.4         | Lua 5.3: see THRIFT-4386 |
+| netstd    | 3.1           | 3.1           | LTS version |
 | nodejs    | 6.16.0        | 10.16.0       |       |
 | ocaml     |               | 4.05.0        | THRIFT-4517: ocaml 4.02.3 on xenial appears broken |
 | perl      | 5.22.1        | 5.26.1        |       |
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index d38901f..79d698f 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -130,7 +130,7 @@
 
 RUN apt-get install -y --no-install-recommends \
 `# dotnet core dependencies` \
-      dotnet-sdk-3.0
+      dotnet-sdk-3.1
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index b63fd58..95a2c78 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -130,7 +130,7 @@
 
 RUN apt-get install -y --no-install-recommends \
 `# dotnet core dependencies` \
-      dotnet-sdk-3.0
+      dotnet-sdk-3.1
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 714a80d..8df0887 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-3.0
+      dotnet-sdk-3.1
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \