add envcheck (names only, no values)

bin/envcheck.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# Print which of the relay vars are set, WITHOUT printing their values. Handy
+# when a script dies with ":? not set" and you cannot tell which one.
+for v in RELAY_ENDPOINT RELAY_USER RELAY_TOKEN CONTAINMENT_CODE; do
+  eval "val=\${$v:-}"
+  if [ -n "$val" ]; then echo "$v: set"; else echo "$v: MISSING"; fi
+done