ns-ovh-sg → ns-do-sg; +ns-gce

We replace `ns-ovh-sg` with `ns-do-sg`; this is a purely financial
decision: `ns-ovh-sg` costs $60/month, $720/year.

`ns-do-sg` (Digital Ocean), is also a Singapore-based DNS server. It's a
basic-regular-2vcpu-4GiB RAM-80GB SSD-4TiB bandwidth for $24/month,
$288/year.

That's a yearly savings of $432.

I had originally overspec'ed the Singapore server because I suspected
that there was a ton of traffic in Asia; I was wrong. It's not even 20%
the traffic of Europe or North America. I am confident the Digital Ocean
server will be able to handle it.

I also reintroduce `ns-gce` as the second server in North America, backing
up `ns-hetzner`. My hope is that `ns-hetzner` carries most of the load,
and `ns-gce` carries the rest, but not so much as to trigger Google
Cloud Platform's (GCP's) expensive bandwidth billing.

| DNS server | Queries / second |
|:-----------|-----------------:|
| ns-hetzner |          10706.4 |
| ns-ovh     |          10802.0 |
| ns-ovh-sg  |           1677.7 |

Docker/sslip.io-dns-server/Dockerfile

@@ -26,7 +26,7 @@ LABEL org.opencontainers.image.authors="Brian Cunnie <brian.cunnie@gmail.com>"
 RUN dnf install -y bind-utils
 
 ARG TARGETARCH # amd64, arm64 (so I can run on AWS graviton2)
-RUN curl -f -L https://github.com/cunnie/sslip.io/releases/download/3.2.7/sslip.io-dns-server-linux-$TARGETARCH \
+RUN curl -f -L https://github.com/cunnie/sslip.io/releases/download/3.2.8/sslip.io-dns-server-linux-$TARGETARCH \
     -o /usr/sbin/sslip.io-dns-server; \
   chmod 755 /usr/sbin/sslip.io-dns-server