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