dockerfile: change to alpine base and cleanup

This makes the image a lot smaller and it's using an official image
as the base to make sure we get security updates etc.

The image size went from:

```
REPOSITORY                      SIZE
gcr.io/google-samples/k8sdocs	837 MB

```

To:

```
REPOSITORY                      SIZE
gcr.io/google-samples/k8sdocs	227 MB
```

Signed-off-by: Jess Frazelle <acidburn@google.com>

staging-container/Dockerfile

@@ -1,13 +1,26 @@
-FROM starefossen/ruby-node:2-4
+FROM alpine:3.3
 
-RUN gem install github-pages
+RUN apk add --no-cache \
+	build-base \
+	ca-certificates \
+	libffi-dev \
+	nodejs \
+	ruby-dev \
+	ruby-nokogiri \
+	zlib-dev
+
+RUN	gem install \
+	bundler \
+	github-pages \
+	io-console \
+	--no-rdoc --no-ri
 
 VOLUME /k8sdocs
 
 EXPOSE 4000
 
+COPY start.sh /start.sh
 WORKDIR /k8sdocs
 
-CMD bundle && jekyll clean && jekyll serve -H 0.0.0.0 -P 4000
-
+CMD [ "/start.sh" ]
 # For instructions, see http://kubernetes.io/editdocs/

staging-container/start.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -e
+set -x
+
+bundle
+bundle exec jekyll clean
+bundle exec jekyll serve -H 0.0.0.0 -P 4000