docker: add rake task for building Docker image.

Add a Rake task for building a Docker image. Usage:

* bin/rails danbooru:docker:build
* sudo docker run --rm -it danbooru

lib/tasks/danbooru.rake

@@ -0,0 +1,10 @@
+namespace :danbooru do
+  namespace :docker do
+    # Note that uncommited changes won't be included in the image; commit
+    # changes first before building the image.
+    desc "Build a Docker image based on latest commit"
+    task :build do
+      system("git archive HEAD | docker build - --build-arg SOURCE_COMMIT=$(git rev-parse HEAD) -t danbooru -f Dockerfile")
+    end
+  end
+end