Add support for passing env vars to the container build env

This is useful if you are using docker and wish to enable buildkit for
example.

Change-Id: I69ae90d945fa3f4eabc10460da936b23c8212858

roles/build-container-image/common.rst

@@ -165,4 +165,10 @@ using the roles described here.
       added with each sibling project.  Note that projects here must
       be listed in ``required-projects``.
 
+.. zuul:rolevar:: container_build_extra_env
+   :type: dict
+
+   A dictionary of key value pairs to add to the container build environment.
+   This may be useful to enable buildkit with docker builds for example.
+
 .. _anchors: https://yaml.org/spec/1.2/spec.html#&%20anchor//

roles/build-container-image/defaults/main.yaml

@@ -1,2 +1,3 @@
 zuul_work_dir: "{{ zuul.project.src_dir }}"
 container_command: podman
+container_build_extra_env: {}

roles/build-container-image/tasks/build.yaml

@@ -52,6 +52,7 @@
     {% endfor -%}
   args:
     chdir: "{{ zuul_work_dir }}/{{ zj_image.context }}"
+  environment: "{{ container_build_extra_env }}"
 
 - name: Cleanup sibling source directory
   file: