Respect Gemfile lockfile in bundle lock

lib/bundler/cli/lock.rb

@@ -69,7 +69,7 @@ def run
           puts definition.to_lock
         else
           file = options[:lockfile]
-          file = file ? Pathname.new(file).expand_path : Bundler.default_lockfile
+          file = file ? Pathname.new(file).expand_path : definition.lockfile
 
           puts "Writing lockfile to #{file}"
           definition.write_lock(file, false)

spec/commands/lock_spec.rb

@@ -177,6 +177,23 @@
     expect(read_lockfile).to eq(expected_lockfile)
   end
 
+  it "writes the lockfile configured in the Gemfile" do
+    build_repo4 do
+      build_gem "foo", "1.0"
+    end
+
+    gemfile <<-G
+      source "https://gem.repo4"
+      gem "foo", "1.0"
+      lockfile "custom.lock"
+    G
+
+    bundle "lock"
+
+    expect(bundled_app("Gemfile.lock")).not_to exist
+    expect(bundled_app("custom.lock")).to exist
+  end
+
   it "prints a lockfile without fetching new checksums if the existing lockfile had no checksums" do
     gemfile_with_rails_weakling_and_foo_from_repo4