only update the necessary packages, (no -Syu)

- move (back) away from updating the whole system, where only a handful
  of packages get's used in the end anyway. this will also drastically
  minify the resulting image size

Dockerfile

@@ -7,6 +7,6 @@ LABEL org.label-schema.build-date=$BUILD_DATE \
       org.label-schema.vcs-url="https://github.com/wULLSnpAXbWZGYDYyhWTKKspEQoaYxXyhoisqHf/docker-archlinux-cdev.git" \
       org.label-schema.vcs-ref=$VCS_REF
 
-RUN pacman -Syu --noconfirm --needed gcc cmake make git valgrind \
+RUN pacman -Sy --noconfirm --needed gcc cmake make git valgrind \
     && pacman -Scc \
     && rm -rfv /var/cache/pacman/* /var/lib/pacman/sync/*