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/cli/build.rb

@@ -35,22 +35,25 @@ class Kamal::Cli::Build < Kamal::Cli::Base
 
     run_locally do
       begin
-        KAMAL.with_verbosity(:debug) do
-          Dir.chdir(KAMAL.config.builder.build_directory) { execute *push }
+        context_hosts = capture_with_info(*KAMAL.builder.context_hosts).split("\n")
+
+        if context_hosts != KAMAL.builder.config_context_hosts
+          warn "Context hosts have changed, so re-creating builder, was: #{context_hosts.join(", ")}], now: #{KAMAL.builder.config_context_hosts.join(", ")}"
+          cli.remove
+          cli.create
         end
       rescue SSHKit::Command::Failed => e
-        if e.message =~ /(no builder)|(no such file or directory)/
-          warn "Missing compatible builder, so creating a new one first"
-
-          if cli.create
-            KAMAL.with_verbosity(:debug) do
-              Dir.chdir(KAMAL.config.builder.build_directory) { execute *push }
-            end
-          end
+        warn "Missing compatible builder, so creating a new one first"
+        if e.message =~ /(context not found|no builder)/
+          cli.create
         else
           raise
         end
       end
+
+      KAMAL.with_verbosity(:debug) do
+        Dir.chdir(KAMAL.config.builder.build_directory) { execute *push }
+      end
     end
   end