Add lxc-docker.install

Adding this install script, which adds a `docker' group, allows users to
be added to the `docker' group, which in turn allows them to run the
docker client without being root.

lxc-docker/PKGBUILD

@@ -9,7 +9,7 @@
 # Maintainer: frio <development@frio.name>
 pkgname=lxc-docker
 pkgver=0.6.3
-pkgrel=1
+pkgrel=2
 pkgdesc="Docker - the Linux container runtime"
 arch=('x86_64')
 url="https://github.com/dotcloud/docker"
@@ -26,6 +26,7 @@ sha1sums=(
     'f6c700a8fe20d1399409046ca74bd52c37678c0c'
     'b040b6fa5465a4ded3239c58bdcc0195a0d196f0'
 )
+install=lxc-docker.install
 
 package() {
   # install systemd service unit

lxc-docker/lxc-docker.install

@@ -0,0 +1,18 @@
+post_install()
+{
+    if ! getent group docker >/dev/null; then
+        groupadd --system docker
+    fi
+}
+
+post_upgrade()
+{
+    post_install $1
+}
+
+post_remove()
+{
+    if getent group docker >/dev/null; then
+        groupdel docker
+    fi
+}