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
 

lib/kamal/commands/builder.rb

@@ -1,7 +1,8 @@
 require "active_support/core_ext/string/filters"
 
 class Kamal::Commands::Builder < Kamal::Commands::Base
-  delegate :create, :remove, :push, :clean, :pull, :info, :validate_image, to: :target
+  delegate :create, :remove, :push, :clean, :pull, :info, :context_hosts, :config_context_hosts, :validate_image,
+    to: :target
 
   include Clone
 

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

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"

lib/kamal/commands/builder/multiarch/remote.rb

@@ -12,6 +12,16 @@ class Kamal::Commands::Builder::Multiarch::Remote < Kamal::Commands::Builder::Mu
       super
   end
 
+  def context_hosts
+    chain \
+      context_host(builder_name_with_arch(local_arch)),
+      context_host(builder_name_with_arch(remote_arch))
+  end
+
+  def config_context_hosts
+    [ local_host, remote_host ].compact
+  end
+
   private
     def builder_name
       super + "-remote"

lib/kamal/commands/builder/native/cached.rb

@@ -1,6 +1,6 @@
 class Kamal::Commands::Builder::Native::Cached < Kamal::Commands::Builder::Native
   def create
-    docker :buildx, :create, "--use", "--driver=docker-container"
+    docker :buildx, :create, "--name", builder_name, "--use", "--driver=docker-container"
   end
 
   def remove
@@ -13,4 +13,13 @@ class Kamal::Commands::Builder::Native::Cached < Kamal::Commands::Builder::Nativ
       *build_options,
       build_context
   end
+
+  def context_hosts
+    docker :buildx, :inspect, builder_name, "> /dev/null"
+  end
+
+  private
+    def builder_name
+      "kamal-#{config.service}-native-cached"
+    end
 end

lib/kamal/commands/builder/native/remote.rb

@@ -26,6 +26,14 @@ class Kamal::Commands::Builder::Native::Remote < Kamal::Commands::Builder::Nativ
     build_context
   end
 
+  def context_hosts
+    context_host(builder_name_with_arch)
+  end
+
+  def config_context_hosts
+    [ remote_host ]
+  end
+
 
   private
     def builder_name