(or "In dash, ... is not supported." when using
trap 'handle_interrupt' SIGINT
trap 'handle_interrupt' INT
POSIX does not allow specifying the "SIG" prefix of a signal name like "INT". Bash and Ksh supports this as an extension.
Simply remove the "SIG" from the signal name.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.