Update base devcontainer image version

Author: stuartleeks

.devcontainer/Dockerfile

@@ -1,6 +1,6 @@
 # [Choice] Debian / Ubuntu version: debian-10, debian-9, ubuntu-20.04, ubuntu-18.04
 # See https://github.com/microsoft/vscode-dev-containers/tree/master/containers/debian
-ARG VARIANT=debian-10
+ARG VARIANT=debian-11
 FROM mcr.microsoft.com/vscode/devcontainers/base:${VARIANT}