[dockerd] Force usage of cgo DNS resolver (#13649)

Go's runtime (and dockerd inherits this) uses own DNS resolver implementation by default on Linux.
It has been observed that there are some DNS resolution issues when executing ```docker pull``` after first boot.

Consider the following script:

```
admin@r-boxer-sw01:~$ while :; do date; cat /etc/resolv.conf; ping -c 1 harbor.mellanox.com; docker pull harbor.mellanox.com/sonic/cpu-report:1.0.0 ; sleep 1; done
Fri 03 Feb 2023 10:06:22 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=5.99 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 5.989/5.989/5.989/0.000 ms
Error response from daemon: Get "https://harbor.mellanox.com/v2/": dial tcp: lookup harbor.mellanox.com on [::1]:53: read udp [::1]:57245->[::1]:53: read: connection refused
Fri 03 Feb 2023 10:06:23 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=5.56 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 5.561/5.561/5.561/0.000 ms
Error response from daemon: Get "https://harbor.mellanox.com/v2/": dial tcp: lookup harbor.mellanox.com on [::1]:53: read udp [::1]:53299->[::1]:53: read: connection refused
Fri 03 Feb 2023 10:06:24 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=5.78 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 5.783/5.783/5.783/0.000 ms
Error response from daemon: Get "https://harbor.mellanox.com/v2/": dial tcp: lookup harbor.mellanox.com on [::1]:53: read udp [::1]:55765->[::1]:53: read: connection refused
Fri 03 Feb 2023 10:06:25 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=7.17 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 7.171/7.171/7.171/0.000 ms
Error response from daemon: Get "https://harbor.mellanox.com/v2/": dial tcp: lookup harbor.mellanox.com on [::1]:53: read udp [::1]:44877->[::1]:53: read: connection refused
Fri 03 Feb 2023 10:06:26 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=5.66 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 5.656/5.656/5.656/0.000 ms
Error response from daemon: Get "https://harbor.mellanox.com/v2/": dial tcp: lookup harbor.mellanox.com on [::1]:53: read udp [::1]:54604->[::1]:53: read: connection refused
Fri 03 Feb 2023 10:06:27 AM UTC
nameserver 10.211.0.124
nameserver 10.211.0.121
nameserver 10.7.77.135
search mtr.labs.mlnx labs.mlnx mlnx lab.mtl.com mtl.com
PING harbor.mellanox.com (10.7.1.117) 56(84) bytes of data.
64 bytes from harbor.mtl.labs.mlnx (10.7.1.117): icmp_seq=1 ttl=53 time=8.22 ms

--- harbor.mellanox.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 8.223/8.223/8.223/0.000 ms
1.0.0: Pulling from sonic/cpu-report
004f1eed87df: Downloading [===================>                               ]   19.3MB/50.43MB
5d6f1e8117db: Download complete
48c2faf66abe: Download complete
234b70d0479d: Downloading [=========>                                         ]  9.363MB/51.84MB
6fa07a00e2f0: Downloading [==>                                                ]   9.51MB/192.4MB
04a31b4508b8: Waiting
e11ae5168189: Waiting
8861a99744cb: Waiting
d59580d95305: Waiting
12b1523494c1: Waiting
d1a4b09e9dbc: Waiting
99f41c3f014f: Waiting
```

While /etc/resolv.conf has the correct content and ping (and any other utility that uses libc's DNS resolution implementation) works correctly
docker is unable to resolve the hostname and falls back to default [::1]:53. This started to happen after PR #13516 has been merged.
As you can see from the log, dockerd is able to pick up the correct /etc/resolv.conf only after 5 sec since first try. This seems to be somehow related to the logic in Go's DNS resolver
https://github.com/golang/go/blob/master/src/net/dnsclient_unix.go#L385.

There have been issues like that reported in docker like:
  - docker/cli#2299
  - docker/cli#2618
  - moby/moby#22398

Since this starts to happen after inclusion of resolvconf package by
above mentioned PR and the fact I can't see any problem with that (ping,
nslookup, etc. works) the choice is made to force dockerd to use cgo
(libc) resolver.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

files/docker/docker.service.conf

@@ -1,3 +1,4 @@
 [Service]
+Environment=GODEBUG=netdns=cgo
 ExecStart=
 ExecStart=/usr/bin/dockerd -H unix:// --storage-driver=overlay2 --bip=240.127.1.1/24 --iptables=false --ipv6=true --fixed-cidr-v6=fd00::/80