Manually set up Dockerfile

.gitpod.Dockerfile

@@ -1,5 +1,23 @@
-FROM gitpod/workspace-full
+FROM gitpod/workspace-base
 
-USER gitpod 
+# for illustration ourposes, this Dockerfile does not inherit from workspace-full but manually sets up Java and Gradle
 
-RUN bash -c ". /home/gitpod/.sdkman/bin/sdkman-init.sh && sdk install java 21.0.4.fx-zulu && sdk default java 21.0.4.fx-zulu"
+USER root
+
+# Install Java 21 (openjdk-21-jdk)
+RUN wget -O- https://apt.corretto.aws/corretto.key | gpg --dearmor | tee /usr/share/keyrings/amazon-corretto-archive-keyring.gpg > /dev/null && \
+    echo 'deb [signed-by=/usr/share/keyrings/amazon-corretto-archive-keyring.gpg] https://apt.corretto.aws stable main' | tee /etc/apt/sources.list.d/corretto.list && \
+    apt-get update && \
+    apt-get install -y java-21-amazon-corretto-jdk && \
+    apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/*
+
+# Install Gradle
+ARG GRADLE_VERSION=8.3
+RUN wget https://services.gradle.org/distributions/gradle-${GRADLE_VERSION}-bin.zip -P /tmp && \
+    unzip -d /opt/gradle /tmp/gradle-${GRADLE_VERSION}-bin.zip && \
+    ln -s /opt/gradle/gradle-${GRADLE_VERSION}/bin/gradle /usr/bin/gradle
+
+# Ensure that Gradle saves all files in /workspace, because files outside that folder are not being backed up when a prebuild finishes or a workspace stops. 
+ENV GRADLE_USER_HOME=/workspace/.gradle/
+
+USER gitpod