Allow custom user and port for builder host

When ssh options are set, they overwrite username and password passed as ssh builder uri. Passing part of uri for ssh-kit is fine, as it then properly extracts username and password and forwards it as host.ssh_options (in which case it's no longer empty)

lib/kamal/cli/build.rb

@@ -114,8 +114,11 @@ class Kamal::Cli::Build < Kamal::Cli::Base
     def connect_to_remote_host(remote_host)
       remote_uri = URI.parse(remote_host)
       if remote_uri.scheme == "ssh"
-        options = { user: remote_uri.user, port: remote_uri.port }.compact
-        on(remote_uri.host, options) do
+        host = SSHKit::Host.new(
+          hostname: remote_uri.host,
+          ssh_options: { user: remote_uri.user, port: remote_uri.port }.compact
+        )
+        on(host, options) do
           execute "true"
         end
       end

lib/kamal/sshkit_with_ext.rb

@@ -80,7 +80,8 @@ class SSHKit::Backend::Netssh
   module LimitConcurrentStartsInstance
     private
       def with_ssh(&block)
-        host.ssh_options = self.class.config.ssh_options.merge(host.ssh_options || {})
+        host.ssh_options = (host.ssh_options || {}).merge({ port: host.port, user: host.user }.compact)
+        host.ssh_options = self.class.config.ssh_options.merge(host.ssh_options)
         self.class.pool.with(
           method(:start_with_concurrency_limit),
           String(host.hostname),