Simplify signal handling (#771)

Avoid implementing a complex signal relaying mechanism in `docker-entrypoint*.sh`
Instead, the shell performs `exec node ...` and the `node` process receives the signals directly.

Related to #560, #575, which were my initial implemetation.

docker-entrypoint.sh

@@ -3,7 +3,7 @@ if ! which -- "${1}"; then
   # first arg is not an executable
   export DISPLAY=:99
   Xvfb "${DISPLAY}" -nolisten unix &
-  exec node /usr/src/app/ -p 80 "$@"
+  exec node /usr/src/app/ "$@"
 fi
 
 exec "$@"

docker-entrypoint_light.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 if ! which -- "${1}"; then
   # first arg is not an executable
-  exec node /usr/src/app/ -p 80 "$@"
+  exec node /usr/src/app/ "$@"
 fi
 
 exec "$@"