build: docker image generation improvements

There are two major changes here. First of all, the image generation now
consist of two separated stages, the build itself and the production
image generation. Secondly, the packages inside image are now installed
as they were at the time of the root image generation (defined by stat
command)

Another side change is that container does not longer ship syncronized
(and out-of-dated) pacman databases; they have to be synced manually

docker/install-aur-package.sh

@@ -4,8 +4,12 @@ set -e
 
 for PACKAGE in "$@"; do
     BUILD_DIR="$(mktemp -d)"
+    # clone the remote source
     git clone https://aur.archlinux.org/"$PACKAGE".git "$BUILD_DIR"
     cd "$BUILD_DIR"
+    # checkout to the image date
+    git checkout "$(git rev-list -1 --before="$(stat -c "%y" "/var/lib/pacman" | cut -d " " -f 1)" master)"
+    # build and install the package
     makepkg --nocheck --noconfirm --install --rmdeps --syncdeps
     cd /
     rm -r "$BUILD_DIR"