Simplify the builders configuration

1. Add driver as an option, defaulting to `docker-container`. For a
   "native" build you can set it to `docker`
2. Set arch as a array of architectures to build for, defaulting to
   `[ "amd64", "arm64" ]` unless you are using the docker driver in
   which case we default to not setting a platform
3. Remote is now just a connection string for the remote builder
4. If remote is set, we only use it for non-local arches, if we are
   only building for the local arch, we'll ignore it.

Examples:

On arm64, build for arm64 locally, amd64 remotely or
On amd64, build for amd64 locally, arm64 remotely:

```yaml
builder:
  remote: ssh://docker@docker-builder
```

On arm64, build amd64 on remote,
On amd64 build locally:

```yaml
builder:
  arch:
    - amd64
  remote:
    host: ssh://docker@docker-builder
```

Build amd64 on local:

```yaml
builder:
  arch:
    - amd64
```

Use docker driver, building for local arch:

```yaml
builder:
  driver: docker
```

lib/kamal/configuration/builder.rb

@@ -19,16 +19,38 @@ class Kamal::Configuration::Builder
     builder_config
   end
 
-  def multiarch?
-    builder_config["multiarch"] != false
+  def remote
+    builder_config["remote"]
   end
 
-  def local?
-    !!builder_config["local"]
+  def arches
+    Array(builder_config.fetch("arch", default_arch))
+  end
+
+  def local_arches
+    @local_arches ||= if remote
+      uname_m = `uname -m`.strip
+      local_arch = uname_m == "x86_64" ? "amd64" : uname_m
+      arches & [ local_arch ]
+    else
+      arches
+    end
+  end
+
+  def remote_arches
+    @remote_arches ||= if remote
+      arches - local_arches
+    else
+      []
+    end
   end
 
   def remote?
-    !!builder_config["remote"]
+    remote_arches.any?
+  end
+
+  def local?
+    arches.empty? || local_arches.any?
   end
 
   def cached?
@@ -59,18 +81,6 @@ class Kamal::Configuration::Builder
     builder_config.fetch("driver", "docker-container")
   end
 
-  def local_arch
-    builder_config["local"]["arch"] if local?
-  end
-
-  def remote_arch
-    builder_config["remote"]["arch"] if remote?
-  end
-
-  def remote_host
-    builder_config["remote"]["host"] if remote?
-  end
-
   def cache_from
     if cached?
       case builder_config["cache"]["type"]
@@ -120,27 +130,14 @@ class Kamal::Configuration::Builder
 
   private
     def valid?
-      if multiarch?
-        if local?
-          raise ArgumentError, "Invalid builder configuration: local configuration, arch required" unless local_arch
-        end
-
-        if remote?
-          raise ArgumentError, "Invalid builder configuration: remote configuration, arch required" unless remote_arch
-          raise ArgumentError, "Invalid builder configuration: remote configuration, arch required" unless remote_host
-        end
-
-        if docker_driver?
-          raise ArgumentError, "Invalid builder configuration: the docker driver does not support multiarch builds"
-        end
-      else
-        raise ArgumentError, "Invalid builder configuration: multiarch must be enabled for local configuration" if local?
-        raise ArgumentError, "Invalid builder configuration: multiarch must be enabled for remote configuration" if remote?
+      if docker_driver?
+        raise ArgumentError, "Invalid builder configuration: the `docker` driver does not not support remote builders" if remote
+        raise ArgumentError, "Invalid builder configuration: the `docker` driver does not not support caching" if cached?
+        raise ArgumentError, "Invalid builder configuration: the `docker` driver does not not support multiple arches" if arches.many?
       end
 
       if @options["cache"] && @options["cache"]["type"]
         raise ArgumentError, "Invalid cache type: #{@options["cache"]["type"]}" unless [ "gha", "registry" ].include?(@options["cache"]["type"])
-        raise ArgumentError, "The docker driver does not support caching" if docker_driver?
       end
     end
 
@@ -179,4 +176,8 @@ class Kamal::Configuration::Builder
     def pwd_sha
       Digest::SHA256.hexdigest(Dir.pwd)[0..12]
     end
+
+    def default_arch
+      docker_driver? ? [] : [ "amd64", "arm64" ]
+    end
 end

lib/kamal/configuration/docs/builder.yml

@@ -3,7 +3,7 @@
 # The builder configuration controls how the application is built with `docker build`
 #
 # If no configuration is specified, Kamal will:
-# 1. Create a buildx context called `kamal-<service>-multiarch`
+# 1. Create a buildx context called `kamal-local-docker-container`, using the docker-container driver
 # 2. Use `docker build` to build a multiarch image for linux/amd64,linux/arm64 with that context
 #
 # See https://kamal-deploy.org/docs/configuration/builder-examples/ for more information
@@ -12,41 +12,29 @@
 #
 # Options go under the builder key in the root configuration.
 builder:
-
-  # Multiarch
-  #
-  # Enables multiarch builds, defaults to `true`
-  multiarch: false
-
   # Driver
   #
   # The build driver to use, defaults to `docker-container`
   driver: docker
 
-  # Local configuration
+  # Arch
   #
-  # The build configuration for local builds, only used if multiarch is enabled (the default)
-  #
-  # If there is no remote configuration, by default we build for amd64 and arm64.
-  # If you only want to build for one architecture, you can specify it here.
-  # The docker socket is optional and uses the default docker host socket when not specified
-  local:
-    arch: amd64
-    host: /var/run/docker.sock
+  # The architectures to build for, defaults to `[ amd64, arm64 ]`
+  # Unless you are using the docker driver, when it defaults to the local architecture
+  # You can set an array or just a single value
+  arch:
+    - amd64
 
   # Remote configuration
   #
-  # The build configuration for remote builds, also only used if multiarch is enabled.
-  # The arch is required and can be either amd64 or arm64.
-  remote:
-    arch: arm64
-    host: ssh://docker@docker-builder
+  # If you have a remote builder, you can configure it here
+  remote: ssh://docker@docker-builder
 
   # Builder cache
   #
   # The type must be either 'gha' or 'registry'
   #
-  # The image is only used for registry cache
+  # The image is only used for registry cache. Not compatible with the docker driver
   cache:
     type: registry
     options: mode=max

lib/kamal/configuration/validator.rb

@@ -28,7 +28,11 @@ class Kamal::Configuration::Validator
             elsif key == "hosts"
               validate_servers! value
             elsif example_value.is_a?(Array)
-              validate_array_of! value, example_value.first.class
+              if key == "arch"
+                validate_array_of_or_type! value, example_value.first.class
+              else
+                validate_array_of! value, example_value.first.class
+              end
             elsif example_value.is_a?(Hash)
               case key.to_s
               when "options", "args"
@@ -71,6 +75,16 @@ class Kamal::Configuration::Validator
       value.is_a?(String) || value.is_a?(Symbol) || value.is_a?(Numeric) || value.is_a?(TrueClass) || value.is_a?(FalseClass)
     end
 
+    def validate_array_of_or_type!(value, type)
+      if value.is_a?(Array)
+        validate_array_of! value, type
+      else
+        validate_type! value, type
+      end
+    rescue Kamal::ConfigurationError
+      type_error(Array, type)
+    end
+
     def validate_array_of!(array, type)
       validate_type! array, Array