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
```

test/integration/main_test.rb

@@ -77,7 +77,7 @@ class MainTest < IntegrationTest
     assert_equal "app-#{version}", config[:service_with_version]
     assert_equal [], config[:volume_args]
     assert_equal({ user: "root", port: 22, keepalive: true, keepalive_interval: 30, log_level: :fatal }, config[:ssh_options])
-    assert_equal({ "multiarch" => false, "args" => { "COMMIT_SHA" => version } }, config[:builder])
+    assert_equal({ "driver" => "docker", "args" => { "COMMIT_SHA" => version } }, config[:builder])
     assert_equal [ "--log-opt", "max-size=\"10m\"" ], config[:logging]
     assert_equal({ "cmd"=>"wget -qO- http://localhost > /dev/null || exit 1", "interval"=>"1s", "max_attempts"=>3, "port"=>3000, "path"=>"/up", "cord"=>"/tmp/kamal-cord", "log_lines"=>50 }, config[:healthcheck])
   end