Gracefully handle signals from Docker or terminal

Start Xvfb wrapper in background and wait for the process to complete.
Because wait exits immediately when a signal for which a trap has been
set, and the signal handler is executed directly after that, we need to
wait again for the background processes to actually finish before exiting.

The signal handler catches INT and TERM and forwards them to the node
process.

The return value from the first wait is stored and sent as exit value.

docker-entrypoint.sh

@@ -2,10 +2,22 @@
 
 set -e
 
+handle() {
+	SIGNAL=$(( $? - 128 ))
+	echo "Caught signal ${SIGNAL}, stopping gracefully"
+	kill -s ${SIGNAL} $(pidof node) 2>/dev/null
+}
+
+trap handle INT TERM
+
 if ! which -- "${1}"; then
   # first arg is not an executable
-  xvfb-run -a --server-args="-screen 0 1024x768x24" -- node /app/ "$@"
+  xvfb-run -a --server-args="-screen 0 1024x768x24" -- node /app/ "$@" &
-  exit $?
+	# Wait exits immediately on signals which have traps set. Store return value and wait
+	# again for all jobs to actually complete before continuing.
+	wait $! || RETVAL=$?
+	wait
+	exit ${RETVAL}
 fi
 
 exec "$@"