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/tasks/test.yml

@@ -24,6 +24,7 @@
     src: '{{ item }}'
     dest: '{{ remote_tmp_dir }}/files/{{ item }}'
   loop:
+    - ArgsDockerfile
     - Dockerfile
     - EtcHostsDockerfile
     - MyDockerfile

tests/integration/targets/docker_image/tasks/tests/options.yml

@@ -27,7 +27,9 @@
     name: "{{ iname }}"
     build:
       path: "{{ remote_tmp_dir }}/files"
+      dockerfile: "ArgsDockerfile"
       args:
+        IMAGE: "{{ docker_test_image_busybox }}"
         TEST1: val1
         TEST2: val2
         TEST3: "True"
@@ -41,7 +43,9 @@
     name: "{{ iname }}"
     build:
       path: "{{ remote_tmp_dir }}/files"
+      dockerfile: "ArgsDockerfile"
       args:
+        IMAGE: "{{ docker_test_image_busybox }}"
         TEST1: val1
         TEST2: val2
         TEST3: "True"