Check that we have valid contexts before building

Load the hosts from the contexts before trying to build.

If there is no context, we'll create one. If there is one but the hosts
don't match we'll re-create.

Where we just have a local context, there won't be any hosts but we
still inspect the builder to check that it exists.

lib/kamal/commands/builder/multiarch.rb

@@ -22,6 +22,10 @@ class Kamal::Commands::Builder::Multiarch < Kamal::Commands::Builder::Base
       build_context
   end
 
+  def context_hosts
+    docker :buildx, :inspect, builder_name, "> /dev/null"
+  end
+
   private
     def builder_name
       "kamal-#{config.service}-multiarch"