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.

dcos-cli/Dockerfile

@@ -1,11 +1,10 @@
 FROM alpine:latest
 MAINTAINER Jessica Frazelle <jess@docker.com>
 
-RUN apk update && apk add \
+RUN apk --no-cache add \
 	ca-certificates \
 	python \
 	py-pip \
-	&& rm -rf /var/cache/apk/* \
 	&& pip install dcoscli
 
 # path to the DCOS CLI binary