THRIFT-5684 upgrade to net7.0:
Client: netstd
Patch: Jens Geyer
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 873dad8..f0dcb90 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -124,10 +124,10 @@
 
 RUN apt-get install -y --no-install-recommends \
       `# dotnet core dependencies` \
-      dotnet-sdk-6.0 \
-      dotnet-runtime-6.0 \
-      aspnetcore-runtime-6.0 \
-      dotnet-apphost-pack-6.0
+      dotnet-sdk-7.0 \
+      dotnet-runtime-7.0 \
+      aspnetcore-runtime-7.0 \
+      dotnet-apphost-pack-7.0
 
 # Erlang dependencies
 ARG ERLANG_OTP_VERSION=23.3.4.11