Add proxy boot_config --publish-ip argument

[phoozle]

Implements basecamp/kamal-proxy#74

Allows for kamal proxy boot_config set --publish-ip=x.x.x.x

Context:
By default Docker will bind to all available interfaces.

[julianrubisch]

This looks great 👏🏻 will try it out asap, thanks!

[phoozle]

@djmb how would you feel about me allowing support for multiple IP addresses to be specified? Would use Thor's :repeatable option

I was just thinking about the folks that may want to bind to a specific interface that supports both IPv4 and IPv6.

Example:

$ kamal proxy boot_config set --publish-host-ip 127.0.0.1 --publish-host-ip '[::1]'
Host my_host_ip: --publish 127.0.0.1:80:80 --publish 127.0.0.1:443:443 --publish [::1]:80:80 --publish [::1]:443:443 --log-opt max-size=10m

[phoozle]

@djmb hey mate, I have just added support for multiple IP addresses. There was a bit of a gotcha with using option default: [ nil ] when also using repeatable: true. Having the default array with nil is needed for the map iteration to run for wildcard interface binding (no IPs specified by user).

option :publish_host_ip, type: :string, repeatable: true, default: [ nil ]
Would produce the following when --publish-host-ip was provided:
(bind_ips => [ nil, "127.0.0.1" ]

So instead, I went ahead and implemented the logic inside #proxy_publish_args with (bind_ips || [ nil ]) and changed publish_host_ip to default to nil instead.

I'm of course open to feedback / alternative solutions on this.

In addition I added a simple IPv4/v6 validator for the argument value (not sure if we want to support this or not).