Add STOPSIGNAL instruction to dockerfiles.

This way, images creators can set the exit signal their programs use.

Signed-off-by: David Calavera <david.calavera@gmail.com>
Upstream-commit: 3781cde61ff10b1d9114ae5b4c5c1d1b2c20a1ee
Component: engine

components/engine/docs/reference/builder.md

@@ -158,6 +158,7 @@ the `Dockerfile`:
 * `USER`
 * `WORKDIR`
 * `VOLUME`
+* `STOPSIGNAL`
 
 as well as:
 
@@ -1012,6 +1013,14 @@ For example you might add something like this:
 
 > **Warning**: The `ONBUILD` instruction may not trigger `FROM` or `MAINTAINER` instructions.
 
+## STOPSIGNAL
+
+	STOPSIGNAL signal
+
+The `STOPSIGNAL` instruction sets the system call signal that will be sent to the container to exit.
+This signal can be a valid unsigned number that matches a position in the kernel's syscall table, for instance 9,
+or a signal name in the format SIGNAME, for instance SIGKILL.
+
 ## Dockerfile examples
 
     # Nginx