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/base.rb

@@ -2,6 +2,8 @@
 class Kamal::Commands::Builder::Base < Kamal::Commands::Base
   class BuilderError < StandardError; end
 
+  ENDPOINT_DOCKER_HOST_INSPECT = "'{{.Endpoints.docker.Host}}'"
+
   delegate :argumentize, to: Kamal::Utils
   delegate :args, :secrets, :dockerfile, :target, :local_arch, :local_host, :remote_arch, :remote_host, :cache_from, :cache_to, :ssh, to: :builder_config
 
@@ -30,6 +32,13 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
       )
   end
 
+  def context_hosts
+    :true
+  end
+
+  def config_context_hosts
+    []
+  end
 
   private
     def build_tags
@@ -74,4 +83,8 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
     def builder_config
       config.builder
     end
+
+    def context_host(builder_name)
+      docker :context, :inspect, builder_name, "--format", ENDPOINT_DOCKER_HOST_INSPECT
+    end
 end