Enable ZeroTier to be optionally included

[jmuchovej]

Modify Dockerfile to "export" two images:

• With ZeroDNS + CoreDNS. (The current jmuchovej/zerodns achieves this.)
• With ZeroDNS + CoreDNS + ZeroTier One client.

Ideally, this can rely on build stages. Unclear if build stages support multiple exports, though.

At worst, this becomes another job in the Docker Builds workflow which uses the latest version of jmuchovej/zerodns but adds ZeroTier. (i.e. this splits the current Dockerfile into base.Dockerfile and with-zerotier.Dockerfile.)