Move documentation generator to Docker

Author: curry684

docs/Gemfile

@@ -0,0 +1,5 @@
+Maintainers with push privileges on `gh-pages` branch can update the
+documentation by running:
+```
+./deploy.sh
+```

docs/Gemfile.lock

@@ -21,7 +21,7 @@ Options:
 
 
 run_build() {
-  bundle exec middleman build --clean
+  docker run --rm --name slate -v $(pwd)/build:/srv/slate/build -v $(pwd)/source:/srv/slate/source slatedocs/slate build
 }
 
 parse_args() {