apk update error from latest alpine

Hi, I tried build docker image from alpine:latest (3.6)
My Dockerfile has asked for an update, but it does not work. See the logs below.

When I tried manually in the container using `docker run -it`, it works.

```sh
Step 1/4 : FROM alpine
---> 76da55c8019d
Step 2/4 : RUN apk update
---> Running in 95477bb69a71
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.6/main: temporary error (try again later)
WARNING: Ignoring APKINDEX.84815163.tar.gz: No such file or directory
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
v3.6.2-190-ga5d68c47df [http://dl-cdn.alpinelinux.org/alpine/v3.6/community]
1 errors; 2895 distinct packages available
```

So I tried build docker image with alpine:3.3
Finally building works like below logs.
Is it problem from version.

```sh
Step 1/4 : FROM alpine:3.3
3.3: Pulling from library/alpine
28db89afc758: Pull complete
Digest: sha256:ae4d16d132e3c93dd09aec45e4c13e9d75ca72e612d221b94fffe2f92de600d7
Status: Downloaded newer image for alpine:3.3
---> c1aae6fe9e10
Step 2/4 : RUN apk update
---> Running in f475604da9c4
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/community/x86_64/APKINDEX.tar.gz
v3.3.3-364-g44b88ac [http://dl-cdn.alpinelinux.org/alpine/v3.3/main]
v3.3.3-294-g70e4c85 [http://dl-cdn.alpinelinux.org/alpine/v3.3/community]
OK: 5860 distinct packages available

```