IO error when running "apk add"

Every time I try to install a package via `apk add` I get IO ERRORs:

``` sh
apk add --update --no-cache ca-certificates
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
WARNING: Ignoring http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz: IO ERROR
ERROR: unsatisfiable constraints:
ca-certificates (missing):
required by: world[ca-certificates]
```

I'm using the alpine:3.6 image on Windows 10 (Docker for Windows - Linux containers)

`apk update` gives similar results:

``` sh
apk update
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: IO ERROR
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
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.6/community: IO ERROR
WARNING: Ignoring APKINDEX.24d64ab1.tar.gz: No such file or directory
2 errors; 11 distinct packages available
```

I don't seem to have a connection problem:

```
curl -v http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
* Trying 151.101.0.249...
* TCP_NODELAY set
* Connected to dl-cdn.alpinelinux.org (151.101.0.249) port 80 (#0)
> GET /alpine/v3.6/community/x86_64/APKINDEX.tar.gz HTTP/1.1
> Host: dl-cdn.alpinelinux.org
> User-Agent: curl/7.56.0
> Accept: */*
>
< HTTP/1.1 200 OK
< X-Bst-Request-Id: bNzl6h:H8B:113240
< X-Bst-Info: t=1508702467,h=03d,p=42804_3138:2_11861,u=402895141,c=2339,v=7.9.58904.98
< Server: nginx
< Content-Type: application/octet-stream
< Last-Modified: Tue, 17 Oct 2017 13:38:29 GMT
< ETag: "59e607d5-5b580"
< Accept-Ranges: bytes
< Accept-Ranges: bytes
< Content-Length: 374144
< Date: Sun, 22 Oct 2017 20:01:07 GMT
< Via: 1.1 varnish, 1.1 hosted.websense 03d
< X-Served-By: cache-cdg8731-CDG
< X-Cache: MISS
< X-Cache-Hits: 0
< X-Timer: S1508702467.312061,VS0,VE32
< Connection: keep-alive
<
```