Improve logging for common failure when using default config (#1801)

jenkins.sh

@@ -4,6 +4,10 @@
 : "${JENKINS_HOME:="/var/jenkins_home"}"
 : "${COPY_REFERENCE_FILE_LOG:="${JENKINS_HOME}/copy_reference_file.log"}"
 : "${REF:="/usr/share/jenkins/ref"}"
+
+if ! [ -r "${JENKINS_HOME}" ] || ! [ -w "${JENKINS_HOME}" ]; then
+        echo "INSTALL WARNING: User: ${USER} missing rw permissions on JENKINS_HOME: ${JENKINS_HOME}"
+fi
 touch "${COPY_REFERENCE_FILE_LOG}" || { echo "Can not write to ${COPY_REFERENCE_FILE_LOG}. Wrong volume permissions?"; exit 1; }
 echo "--- Copying files at $(date)" >> "$COPY_REFERENCE_FILE_LOG"
 find "${REF}" \( -type f -o -type l \) -exec bash -c '. /usr/local/bin/jenkins-support; for arg; do copy_reference_file "$arg"; done' _ {} +