diff --git a/roles/build-docker-image/common.rst b/roles/build-docker-image/common.rst
index 95d37adcf..8a5cc977d 100644
--- a/roles/build-docker-image/common.rst
+++ b/roles/build-docker-image/common.rst
@@ -119,4 +119,15 @@ using this role.
 
       A list of tags to be added to the image when promoted.
 
+   .. zuul:rolevar:: siblings
+      :type: list
+      :default: []
+
+      A list of sibling projects to be copied into
+      ``{{zuul_work_dir}}/.zuul-siblings``.  This can be useful to
+      collect multiple projects to be installed within the same Docker
+      context.  A ``-build-arg`` called ``ZUUL_SIBLINGS`` will be
+      added with each sibling project.  Note that projects here must
+      be listed in ``required-projects``.
+
 .. _anchors: https://yaml.org/spec/1.2/spec.html#&%20anchor//
diff --git a/roles/build-docker-image/tasks/build.yaml b/roles/build-docker-image/tasks/build.yaml
new file mode 100644
index 000000000..99eac9b19
--- /dev/null
+++ b/roles/build-docker-image/tasks/build.yaml
@@ -0,0 +1,51 @@
+- name: Check sibling directory
+  stat:
+    path: '{{ zuul_work_dir }}/.zuul-siblings'
+  register: _dot_zuul_siblings
+
+# This should have been cleaned up; multiple builds may specify
+# different siblings to include so we need to start fresh.
+- name: Check for clean build
+  assert:
+    that: not _dot_zuul_siblings.stat.exists
+
+- name: Create sibling source directory
+  file:
+    path: '{{ zuul_work_dir }}/.zuul-siblings'
+    state: directory
+    mode: 0755
+  when: item.siblings is defined
+
+# NOTE(ianw): could use recursive copy: with remote_src, but it's
+# Ansible 2.8 only.  take the simple approach.
+- name: Copy sibling source directories
+  command: 'cp -r ~/src/{{ sibling }} {{ zuul_work_dir }}/.zuul-siblings/'
+  loop: '{{ item.siblings }}'
+  loop_control:
+    loop_var: sibling
+  when: item.siblings is defined
+
+- name: Build a docker image
+  command: >-
+    docker build {{ item.path | default('.') }} -f {{ item.dockerfile | default(docker_dockerfile) }}
+    {% if item.target | default(false) -%}
+      --target {{ item.target }}
+    {% endif -%}
+    {% for build_arg in item.build_args | default([]) -%}
+      --build-arg {{ build_arg }}
+    {% endfor -%}
+    {% if item.siblings | default(false) -%}
+      --build-arg "ZUUL_SIBLINGS={{ item.siblings | join(' ') }}"
+    {% endif -%}
+    {% for tag in item.tags | default(['latest']) -%}
+    --tag {{ item.repository }}:change_{{ zuul.change }}_{{ tag }}
+    --tag {{ item.repository }}:{{ tag }}
+    {% endfor -%}
+  args:
+    chdir: "{{ zuul_work_dir }}/{{ item.context }}"
+
+- name: Cleanup sibling source directory
+  file:
+    path: '{{ zuul_work_dir }}/.zuul-siblings'
+    state: absent
+
diff --git a/roles/build-docker-image/tasks/main.yaml b/roles/build-docker-image/tasks/main.yaml
index 4dceac00a..c5d089880 100644
--- a/roles/build-docker-image/tasks/main.yaml
+++ b/roles/build-docker-image/tasks/main.yaml
@@ -4,22 +4,11 @@
   set_fact:
     buildset_registry: "{{ (lookup('file', zuul.executor.work_root + '/results.json') | from_json)['buildset_registry'] }}"
   ignore_errors: true
-- name: Build a docker image
-  command: >-
-    docker build {{ item.path | default('.') }} -f {{ item.dockerfile | default(docker_dockerfile) }}
-    {% if item.target | default(false) -%}
-      --target {{ item.target }}
-    {% endif -%}
-    {% for build_arg in item.build_args | default([]) -%}
-      --build-arg {{ build_arg }}
-    {% endfor -%}
-    {% for tag in item.tags | default(['latest']) -%}
-    --tag {{ item.repository }}:change_{{ zuul.change }}_{{ tag }}
-    --tag {{ item.repository }}:{{ tag }}
-    {% endfor -%}
-  args:
-    chdir: "{{ zuul_work_dir }}/{{ item.context }}"
+
+- name: Build docker images
+  include_tasks: build.yaml
   loop: "{{ docker_images }}"
+
 # Docker doesn't understand docker push [1234:5678::]:5000/image/path:tag
 # so we set up /etc/hosts with a registry alias name to support ipv6 and 4.
 - name: Configure /etc/hosts for buildset_registry to workaround docker not understanding ipv6 addresses
diff --git a/test-playbooks/registry/test-registry.yaml b/test-playbooks/registry/test-registry.yaml
index da1251908..c85ae8ea5 100644
--- a/test-playbooks/registry/test-registry.yaml
+++ b/test-playbooks/registry/test-registry.yaml
@@ -122,12 +122,25 @@
 
 - hosts: builder
   name: Test building a docker image
-  roles:
-    - role: build-docker-image
+  tasks:
+
+    - name: Create fake sibling projects
+      command: >-
+        mkdir -p src/opendev.org/fake-sibling-1 &&
+        mkdir -p src/opendev.org/fake-sibling-2 &&
+        touch  src/opendev.org/fake-sibling-1/file &&
+        touch  src/opendev.org/fake-sibling-2/file
+
+    - name: Build docker image
+      include_role:
+        name: build-docker-image
       vars:
         docker_images:
           - context: test-playbooks/registry/docker
             repository: downstream/image
+            siblings:
+              - opendev.org/fake-sibling-1
+              - opendev.org/fake-sibling-2
 
 - hosts: executor
   name: Test pushing to the intermediate registry