docker_image_build: allow to specify multiple platforms, allow to specify secrets and outputs (#852)

* Add note on idempotency.

* Make platform a list of strings.

* Support specifying secrets.

* Add test for secrets.

* Support specifying outputs.

* Ignore invalid choices syntax for ansible-core <= 2.16.

It actually works with ansible-core 2.14+ (though not with <= 2.13),
but the sanity tests only accept it from 2.17 on.

* Only use --secret with type=env for buildx 0.6.0+, and multiple --output for buildx 0.13.0+.

tests/integration/targets/docker_image_build/tasks/test.yml

@@ -28,12 +28,24 @@
     - Dockerfile
     - EtcHostsDockerfile
     - MyDockerfile
+    - SecretsDockerfile
     - StagedDockerfile
 
 - debug:
     msg: "Has buildx plugin: {{ docker_has_buildx }}"
 
 - block:
+    - name: Determine plugin versions
+      command: docker info -f '{{ "{{" }}json .ClientInfo.Plugins{{ "}}" }}'
+      register: plugin_versions
+
+    - name: Determine buildx plugin version
+      set_fact:
+        buildx_version: >-
+          {{
+            (plugin_versions.stdout | from_json | selectattr('Name', 'eq', 'buildx') | map(attribute='Version') | first).lstrip('v')
+          }}
+
     - include_tasks: run-test.yml
       with_fileglob:
         - "tests/*.yml"