docker: prefix role vars

These probably should have been prefixed to start with.  The roles
are brand new, not publicised, and likely not widely used.  I think
we can merge this without announcement or deprecation.

Change-Id: I7825ef6fee1325b6d4fcc179032652eb5530d016

roles/build-docker-image/common.rst

@@ -42,7 +42,7 @@ using this role.
    The project directory.  Serves as the base for
    :zuul:rolevar:`build-docker-image.docker_images.context`.
 
-.. zuul:rolevar:: credentials
+.. zuul:rolevar:: docker_credentials
    :type: dict
 
    This is only required for the upload and promote roles.  This is

roles/build-docker-image/tasks/main.yaml

@@ -10,4 +10,4 @@
     --tag {{ item.repository }}:change_{{ zuul.change }}
   args:
     chdir: "{{ zuul_work_dir }}/{{ item.context }}"
-  loop: "{{ images }}"
+  loop: "{{ docker_images }}"

roles/promote-docker-image/tasks/main.yaml

@@ -5,16 +5,16 @@
     url: "https://hub.docker.com/v2/users/login/"
     body_format: json
     body:
-      username: "{{ credentials.username }}"
+      username: "{{ docker_credentials.username }}"
-      password: "{{ credentials.password }}"
+      password: "{{ docker_credentials.password }}"
   register: jwt_token
 - name: Promote image
-  loop: "{{ images }}"
+  loop: "{{ docker_images }}"
   loop_control:
     loop_var: image
   include_tasks: promote-retag.yaml
 - name: Delete obsolete tags
-  loop: "{{ images }}"
+  loop: "{{ docker_images }}"
   loop_control:
     loop_var: image
   include_tasks: promote-cleanup.yaml

roles/promote-docker-image/tasks/promote-retag.yaml

@@ -2,8 +2,8 @@
   no_log: true
   uri:
     url: "https://auth.docker.io/token?service=registry.docker.io&scope=repository:{{ image.repository }}:pull,push"
-    user: "{{ credentials.username }}"
+    user: "{{ docker_credentials.username }}"
-    password: "{{ credentials.password }}"
+    password: "{{ docker_credentials.password }}"
     force_basic_auth: true
   register: token
 - name: Get manifest

roles/upload-docker-image/tasks/main.yaml

@@ -1,6 +1,6 @@
 - name: Log in to dockerhub
-  command: "docker login -u {{ credentials.username }} -p {{ credentials.password }}"
+  command: "docker login -u {{ docker_credentials.username }} -p {{ docker_credentials.password }}"
   no_log: true
 - name: Upload to dockerhub
   command: "docker push {{ item.repository }}:change_{{ zuul.change }}"
-  loop: "{{ images }}"
+  loop: "{{ docker_images }}"