Based on this comment (https://github.com/drone/drone/issues/2189#issuecomment-327017098), SIGINT is definitely supported.
How about SIGTERM?
In Kubernetes, container termination is signaled with SIGTERM for graceful handling (no SIGINT unfortunately). It is the preferred way to signal processes to exit after all (https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html).