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

This closes #1909
diff --git a/build/docker/README.md b/build/docker/README.md
index 08c1372..e33c4e0 100644
--- a/build/docker/README.md
+++ b/build/docker/README.md
@@ -177,7 +177,7 @@
 | d         | 2.075.1       | 2.087.0       |       |
 | dart      | 2.0.0         | 2.4.0         |       |
 | delphi    |               |               | Not in CI |
-| dotnet    | 2.1.503       | 2.2.300       |       |
+| dotnet    | 3.0           | 3.0           |       |
 | erlang    | 18.3          | 22.0          |       |
 | go        | 1.10.8        | 1.12.6        |       |
 | haskell   | 7.10.3        | 8.0.2         |       |
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 5f9833f..d38901f 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-2.2
+      dotnet-sdk-3.0
 
 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 b017c4e..b63fd58 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-2.2
+      dotnet-sdk-3.0
 
 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 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` \