alpine.gliderlabs.com DNS SERVFAIL

Reproduce:

```bash
docker run gliderlabs/alpine apk update
```

Result:

```
fetch http://alpine.gliderlabs.com/alpine/v3.9/main/x86_64/APKINDEX.tar.gz
ERROR: http://alpine.gliderlabs.com/alpine/v3.9/main: temporary error (try again later)
WARNING: Ignoring APKINDEX.4b0f3aca.tar.gz: No such file or directory
fetch http://alpine.gliderlabs.com/alpine/v3.9/community/x86_64/APKINDEX.tar.gz
ERROR: http://alpine.gliderlabs.com/alpine/v3.9/community: temporary error (try again later)
WARNING: Ignoring APKINDEX.e20d04a5.tar.gz: No such file or directory
2 errors; 11 distinct packages available
```

nslookup result (DNS Resolver: 1.1.1.1 / 8.8.8.8):

```
nslookup alpine.gliderlabs.com
;; Got SERVFAIL reply from 1.1.1.1, trying next server
Server: 8.8.8.8
Address: 8.8.8.8#53

** server can't find alpine.gliderlabs.com: SERVFAIL
```