gr-gsm-master/Dockerfile: fix full system upgrade

Thanks to Piotr, it was discovered that the image building
process fails during full system upgrade:

  RUN    pacman -Syu

with the following output:

  looking for conflicting packages...
  Packages (2) ca-certificates-mozilla-*  p11-kit-*

  ...

  :: Proceed with installation? [Y/n]
  The command '/bin/sh -c pacman -Syu' returned a non-zero code: 1

Looks like the '--noconfirm' flag was missing.

Change-Id: Icb3ccb9a9531686efcebb3e277421c4c05387b09

gr-gsm-master/Dockerfile

@@ -17,7 +17,7 @@ ENV	BUILD_DEPS \
 	gcc
 
 # Sync pacman
-RUN	pacman -Syu
+RUN	pacman -Syyu --noconfirm
 
 # Install persistent packages
 RUN	pacman -S --needed --noconfirm \