Add support for per-image build-arg settings

Change-Id: Ia14397389be0c7178c1879844cd675a3d291a3d0

.zuul.yaml

@@ -69,6 +69,10 @@
             docker build. Useful for building images with a Dockerfile
             in the context directory but a source repository elsewhere.
 
+         .. zuul:jobvar:: build_args
+
+            Optional; if supplied, a list of values to pass to the docker
+            ``--build-arg`` parameter.
     abstract: true
     pre-run: playbooks/zuul/build-image/pre.yaml
     run: playbooks/zuul/build-image/run.yaml

docker/gitea/Dockerfile

@@ -112,4 +112,4 @@ EXPOSE 22
 VOLUME ["/data"]
 ENTRYPOINT ["/usr/bin/entrypoint"]
 CMD ["/usr/sbin/sshd", "-D"]
-# this comment is here to perform a test run of the job....
+# this comment is here to perform a test run of the job.....

playbooks/zuul/build-image/run.yaml

@@ -1,7 +1,15 @@
 - hosts: all
   tasks:
     - name: Build a docker image
-      command: "docker build {{ item.path | default('.') }} -f Dockerfile {{ target | default(false) | ternary('--target ', '') }}{{ target | default('') }} --tag {{ item.repository }}:change_{{ zuul.change }}"
+      command: >-
+        docker build {{ item.path | default('.') }} -f Dockerfile
+        {% if target | default(false) -%}
+          --target {{ target }}
+        {% endif -%}
+        {% for build_arg in item.build_args | default([]) -%}
+          --build-arg {{ build_arg }}
+        {% endfor -%}
+        --tag {{ item.repository }}:change_{{ zuul.change }}
       args:
         chdir: "{{ zuul.project.src_dir }}/{{ item.context }}"
       loop: "{{ images }}"