Add auto-update

Signed-off-by: Knut Ahlers <knut@ahlers.me>

.travis.yml

@@ -0,0 +1,7 @@
+sudo: required
+
+services:
+  - docker
+
+script:
+  - bash update.sh

update.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+set -euo pipefail
+
+### ---- ###
+
+echo "Switch back to master"
+git checkout master
+git reset --hard origin/master
+
+### ---- ###
+
+version=$(curl -s "https://lv.luzifer.io/catalog-api/yggdrasil/latest.txt?p=version")
+grep -q "YGGDRASIL_VERSION=${version} " Dockerfile && exit 0 || echo "Update required"
+
+sed -Ei \
+	-e "s/YGGDRASIL_VERSION=[0-9.]+/YGGDRASIL_VERSION=${version}/" \
+	Dockerfile
+
+### ---- ###
+
+echo "Testing build..."
+docker build .
+
+### ---- ###
+
+echo "Updating repository..."
+git add Dockerfile
+git -c user.name='Travis Automated Update' -c user.email='travis@luzifer.io' \
+	commit -m "Yggdrasil ${version}"
+git tag v${version}
+
+git push -q https://${GH_USER}:${GH_TOKEN}@github.com/luzifer-docker/yggdrasil master --tags