chore(Dockerfile) unify JENKINS_AGENT_JDK_MAJOR build arguments (#120)

Co-authored-by: Jenkins Infra Bot (updatecli) <60776566+jenkins-infra-bot@users.noreply.github.com>

Dockerfile

@@ -102,7 +102,7 @@ ENV PATH "${JAVA_HOME}/bin:${PATH}"
 ## Note: when using the same major versions, the temurin JDK overrides the agent JDK.
 ##    We need to keep this behavior as both JDK can differ. The long term solution is to switch this image to the "all in one".
 # Repeat ARG to scope it in this stage
-ARG JENKINS_AGENT_JDK_MAJOR
+ARG JENKINS_AGENT_JDK_MAJOR=17
 COPY --from=jenkins-agent /opt/java/openjdk /opt/jdk-"${JENKINS_AGENT_JDK_MAJOR}"
 COPY --from=jdk /opt/java/openjdk ${JAVA_HOME}