Update Java from 17 to 21

dodona-java17.dockerfile → dodona-java21.dockerfile

@@ -1,13 +1,13 @@
-FROM openjdk:17-slim
+FROM eclipse-temurin:21-jdk-alpine
 
 # Install jq for json querying in bash
-RUN apt-get update && apt-get install -y --no-install-recommends jq=1.6-2.1 \
- && rm -rf /var/lib/apt/lists/* \
+RUN apk add --update-cache jq \
  # Make sure the students can't find our secret path, which is mounted in
  # /mnt with a secure random name.
  && chmod 711 /mnt \
  # Add the user which will run the student's code and the judge.
- && useradd -m runner
+ && adduser -S runner \
+ && rm -rf /var/cache/apk/*
 
 # As the runner user
 USER runner