Use `apk --no-cache' on alpine images (#144)

This enables us to remove the following patterns that required
a `rm -rf /var/cache/apk`:

- `apk update`
- `apk add --update`
- `apk add --update-cache`

Supported since alpine 3.3.

ab/Dockerfile

@@ -7,11 +7,10 @@
 FROM alpine:latest
 MAINTAINER Jessica Frazelle <jess@docker.com>
 
-RUN apk update && apk add \
+RUN apk --no-cache add \
 	apache2-ssl \
 	apache2-utils \
 	ca-certificates \
-	htop \
-	&& rm -rf /var/cache/apk/*
+	htop
 
 ENTRYPOINT [ "ab" ]