Added a makefile and an example of a derived image

Dockerfile.example

@@ -0,0 +1,2 @@
+# use latest or one of the published tags as your base
+FROM neochrome/bastion:latest

Makefile

@@ -0,0 +1,19 @@
+.DEFAULT_GOAL := build
+
+BUILD_TAG=neochrome/bastion:latest
+EXAMPLE_TAG=neochrome/bastion:example
+
+build:
+	@docker build -t $(BUILD_TAG) .
+
+example: build
+	@docker build -t $(EXAMPLE_TAG) -f Dockerfile.example .
+
+example-run: example
+	@docker run --rm -it -p 2222:22 -v $$HOME/.ssh/id_rsa.pub:/bastion/authorized_keys $(EXAMPLE_TAG)
+
+example-test:
+	@ssh bastion@localhost -p 2222 /bin/true
+
+clean:
+	@docker rmi -f $(EXAMPLE_TAG) $(BUILD_TAG)

README.md

@@ -8,6 +8,7 @@ to `/bastion`. I.e, one must connect as the `bastion` user like so:
 ```
 $ ssh bastion@hostname
 ```
+Please see [Dockerfile.example](Dockerfile.example) for a minimal example of this.
 
 ### google-authenticator
 Upon first connection `google-authenticator` will be run in order to