Fix docker_image's build.args (#456)

* Add tests for build.args.

* Fix bug: store build args in correct dict

* Add changelog fragment.

* Update copyright notice.

tests/integration/targets/docker_image/templates/ArgsDockerfile

@@ -0,0 +1,13 @@
+# Copyright (c) 2022, Felix Fontein
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+ARG IMAGE
+ARG TEST1
+ARG TEST2
+ARG TEST3
+
+FROM ${IMAGE}
+ENV foo /bar
+WORKDIR ${foo}
+RUN echo "${TEST1} - ${TEST2} - ${TEST3}"