Update entrypoint.sh to use POSIX-compliant date command for next run time calculation (#19)

Author: jaydrogers

src/entrypoint.sh

@@ -204,7 +204,7 @@ else
         # POSIX-compliant way to show next run time
         current_timestamp=$(date +%s)
         next_timestamp=$((current_timestamp + RENEWAL_INTERVAL))
-        next_run=$(date -r "$next_timestamp" '+%Y-%m-%d %H:%M:%S %z' 2>/dev/null || date '+%Y-%m-%d %H:%M:%S %z')
+        next_run=$(date -d "@$next_timestamp" '+%Y-%m-%d %H:%M:%S %z' 2>/dev/null || date '+%Y-%m-%d %H:%M:%S %z')
         echo "Next certificate renewal check will be at ${next_run}"
 
         # Store PID of sleep process and wait for it