buildctl-daemonless.sh: show log on "could not connect to ..."

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

examples/buildctl-daemonless/buildctl-daemonless.sh

@@ -45,6 +45,8 @@ waitForBuildkitd() {
     until $BUILDCTL --addr=$addr debug workers >/dev/null 2>&1; do
         if [ $try -gt $max ]; then
             echo >&2 "could not connect to $addr after $max trials"
+            echo >&2 "========== log =========="
+            cat >&2 $tmp/log
             exit 1
         fi
         sleep $(awk "BEGIN{print (100 + $try * 20) * 0.001}")