Merge pull request #41135 from SimenB/no-cache-alpine-image

Automatic merge from submit-queue

chore (samples): Don't cache apks at all in Dockerfiles

**What this PR does / why we need it**:
It simplifies the sample Dockerfiles somewhat.

https://github.com/gliderlabs/docker-alpine/blob/master/docs/usage.md#disabling-cache

**Special notes for your reviewer**:
While I'm here, any reason the Redis sample doesn't use the official Redis image from Dockerhub?

**Release note**:

```release-note
```

examples/storage/redis/image/Dockerfile

@@ -14,7 +14,7 @@
 
 FROM alpine:3.4
 
-RUN apk add -U redis sed bash && rm -rf /var/cache/apk/*
+RUN apk add --no-cache redis sed bash
 
 COPY redis-master.conf /redis-master/redis.conf
 COPY redis-slave.conf /redis-slave/redis.conf

test/images/dnsutils/Dockerfile

@@ -14,5 +14,4 @@
 
 FROM alpine
 
-RUN apk update --no-cache
-RUN apk add bind-tools
+RUN apk add --no-cache bind-tools