Update Dockerfile and add toolchain download

This commit updates the Dockerfile to use JDK21 and adds configuration
to auto-download a JDK.

The Dockerfile needs to be updated to JDK21 because the autodownload
cannot find a JDK21 for Ubuntu on M1 Macs (so needs to use one set
up in the Docker image) for tutorial 3

.devcontainer/Dockerfile

-FROM openjdk:17-jdk-slim-bullseye
+FROM openjdk:21-jdk-slim-bullseye
 
 # Install some basic development utilities
 RUN apt-get update && apt-get install -y \ 

settings.gradle

@@ -4,5 +4,9 @@ pluginManagement {
     }
 }
 
+plugins {
+    id("org.gradle.toolchains.foojay-resolver-convention") version "0.8.0"
+}
+
 rootProject.name = 'dotsAndBoxes'