Build from within a git clone by default

Docker does not respect the .dockerignore file when building from a tar.

Instead by default we'll make a local clone into a tmp directory and
build from there. Subsequent builds will reset the clone to match the
checkout.

Compared to building directly in the repo, we'll have reproducible
builds.

Compared to using a git archive:
1. .dockerignore is respected
2. We'll have faster builds - docker can be smarter about caching the
build context on subsequent builds from a directory

To build from the repo directly, set the build context to "." in the
config.

If there are uncommitted changes, we'll warn about them either being
included or ignored depending on whether we build from the clone.

lib/kamal/configuration/builder.rb

@@ -3,6 +3,8 @@ class Kamal::Configuration::Builder
     @options = config.raw_config.builder || {}
     @image = config.image
     @server = config.registry["server"]
+    @service = config.service
+    @destination = config.destination
 
     valid?
   end
@@ -44,7 +46,7 @@ class Kamal::Configuration::Builder
   end
 
   def context
-    @options["context"] || (git_archive? ? "-" : ".")
+    @options["context"] || "."
   end
 
   def local_arch
@@ -89,10 +91,23 @@ class Kamal::Configuration::Builder
     @options["ssh"]
   end
 
-  def git_archive?
+  def git_clone?
     Kamal::Git.used? && @options["context"].nil?
   end
 
+  def clone_directory
+    @clone_directory ||= File.join Dir.tmpdir, "kamal-clones", [ @service, @destination, pwd_sha ].compact.join("-")
+  end
+
+  def build_directory
+    @build_directory ||=
+      if git_clone?
+        File.join clone_directory, repo_basename, repo_relative_pwd
+      else
+        "."
+      end
+  end
+
   private
     def valid?
       if @options["cache"] && @options["cache"]["type"]
@@ -123,4 +138,16 @@ class Kamal::Configuration::Builder
     def cache_to_config_for_registry
       [ "type=registry", @options["cache"]&.fetch("options", nil), "ref=#{cache_image_ref}" ].compact.join(",")
     end
+
+    def repo_basename
+      File.basename(Kamal::Git.root)
+    end
+
+    def repo_relative_pwd
+      Dir.pwd.delete_prefix(Kamal::Git.root)
+    end
+
+    def pwd_sha
+      Digest::SHA256.hexdigest(Dir.pwd)[0..12]
+    end
 end