Combine multiarch and native/cache builders

Combine the two builders, as they are almost identical. The only
difference was whether the platforms were set.

The native cached builder wasn't using the context it created, so now
we do.

We'll set the driver to `docker-container` - it seems to be the default
but the Docker docs claim it is `docker`.

test/commands/builder_test.rb

@@ -7,9 +7,9 @@ class CommandsBuilderTest < ActiveSupport::TestCase
 
   test "target multiarch by default" do
     builder = new_builder_command(builder: { "cache" => { "type" => "gha" } })
-    assert_equal "multiarch", builder.name
+    assert_equal "local", builder.name
     assert_equal \
-      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-app-multiarch -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile .",
+      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-local -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile .",
       builder.push.join(" ")
   end
 
@@ -23,9 +23,9 @@ class CommandsBuilderTest < ActiveSupport::TestCase
 
   test "target native cached when multiarch is off and cache is set" do
     builder = new_builder_command(builder: { "multiarch" => false, "cache" => { "type" => "gha" } })
-    assert_equal "native/cached", builder.name
+    assert_equal "local", builder.name
     assert_equal \
-      "docker buildx build --push -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile .",
+      "docker buildx build --push --builder kamal-local -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile .",
       builder.push.join(" ")
   end
 
@@ -39,9 +39,9 @@ class CommandsBuilderTest < ActiveSupport::TestCase
 
   test "target multiarch local when arch is set" do
     builder = new_builder_command(builder: { "local" => { "arch" => "amd64" } })
-    assert_equal "multiarch", builder.name
+    assert_equal "local", builder.name
     assert_equal \
-      "docker buildx build --push --platform linux/amd64 --builder kamal-app-multiarch -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile .",
+      "docker buildx build --push --platform linux/amd64 --builder kamal-local -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile .",
       builder.push.join(" ")
   end
 
@@ -93,7 +93,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
   test "build context" do
     builder = new_builder_command(builder: { "context" => ".." })
     assert_equal \
-      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-app-multiarch -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile ..",
+      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-local -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile ..",
       builder.push.join(" ")
   end
 
@@ -107,7 +107,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
   test "multiarch push with build args" do
     builder = new_builder_command(builder: { "args" => { "a" => 1, "b" => 2 } })
     assert_equal \
-      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-app-multiarch -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --build-arg a=\"1\" --build-arg b=\"2\" --file Dockerfile .",
+      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-local -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --build-arg a=\"1\" --build-arg b=\"2\" --file Dockerfile .",
       builder.push.join(" ")
   end
 
@@ -133,7 +133,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
   test "multiarch context build" do
     builder = new_builder_command(builder: { "context" => "./foo" })
     assert_equal \
-      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-app-multiarch -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile ./foo",
+      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-local -t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile ./foo",
       builder.push.join(" ")
   end
 
@@ -147,7 +147,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
   test "cached context build" do
     builder = new_builder_command(builder: { "multiarch" => false, "context" => "./foo", "cache" => { "type" => "gha" } })
     assert_equal \
-      "docker buildx build --push -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile ./foo",
+      "docker buildx build --push --builder kamal-local -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile ./foo",
       builder.push.join(" ")
   end
 
@@ -160,7 +160,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
 
   test "multiarch context hosts" do
     command = new_builder_command
-    assert_equal "docker buildx inspect kamal-app-multiarch > /dev/null", command.context_hosts.join(" ")
+    assert_equal "docker buildx inspect kamal-local > /dev/null", command.context_hosts.join(" ")
     assert_equal "", command.config_context_hosts.join(" ")
   end
 
@@ -172,7 +172,7 @@ class CommandsBuilderTest < ActiveSupport::TestCase
 
   test "native cached context hosts" do
     command = new_builder_command(builder: { "multiarch" => false, "cache" => { "type" => "registry" } })
-    assert_equal "docker buildx inspect kamal-app-native-cached > /dev/null", command.context_hosts.join(" ")
+    assert_equal "docker buildx inspect kamal-local > /dev/null", command.context_hosts.join(" ")
     assert_equal "", command.config_context_hosts.join(" ")
   end