Build from a git archive

Building directly from a checkout will pull in uncommitted files to or
more sneakily files that are git ignored, but not docker ignored.

To avoid this, we'll add an option to build from a git archive of HEAD
instead. Docker doesn't provide a way to build directly from a git
repo, so instead we create a tarball of the current HEAD with git
archive and pipe it into the build command.

When building from a git archive, we'll still display the warning about
uncommitted changes, but we won't add the `_uncommitted_...` suffix to
the container name as they won't be included in the build.

Perhaps this should be the default, but we'll leave that decision for
now.

test/configuration_test.rb

@@ -106,6 +106,16 @@ class ConfigurationTest < ActiveSupport::TestCase
     assert_match /^git-version_uncommitted_[0-9a-f]{16}$/, @config.version
   end
 
+  test "version from git archive uncommitted" do
+    ENV.delete("VERSION")
+
+    config = Kamal::Configuration.new(@deploy.tap { |c| c[:builder] = { "git_archive" => true } })
+
+    Kamal::Git.expects(:revision).returns("git-version")
+    Kamal::Git.expects(:uncommitted_changes).returns("M   file\n")
+    assert_equal "git-version", config.version
+  end
+
   test "version from env" do
     ENV["VERSION"] = "env-version"
     assert_equal "env-version", @config.version