computer-playbook/docs/contributing/code/role/dockerfile.md

Dockerfile

This page is the SPOT for role-local Dockerfiles. Use this page for placement rules, variable handling, and build wiring. For the agent-side review workflow during development, see Development.

Placement

• You MUST place role-local Dockerfiles at files/Dockerfile.
• You MUST NOT use templates/Dockerfile.j2 unless the Dockerfile requires Jinja2 control-flow logic (e.g. {% if %}, {% for %}).
• sys-svc-compose discovers the Dockerfile automatically by checking templates/Dockerfile.j2 first and then files/Dockerfile. Both are rendered through the Ansible template module.

Variables

• You MUST NOT hard-code values that come from config/main.yml or vars/main.yml directly in files/Dockerfile.
• You MUST declare each external value as a Docker ARG without a default value so the build always requires the value to be passed explicitly.
• You MUST pass every ARG via the args: block in templates/compose.yml.j2, directly after the {{ lookup('template', 'roles/sys-svc-container/templates/build.yml.j2') }} call.
• The vars/main.yml of the role MUST define the variables referenced in args: by reading them from config/main.yml through the lookup('config', ...) filter. This keeps config/main.yml as the single source of truth.

Example files/Dockerfile:

ARG APP_IMAGE
ARG APP_VERSION
FROM ${APP_IMAGE}:${APP_VERSION}

Example templates/compose.yml.j2 wiring:

    {{ lookup('template', 'roles/sys-svc-container/templates/build.yml.j2') | indent(4) }}
      args:
        APP_IMAGE:   "{{ APP_IMAGE }}"
        APP_VERSION: "{{ APP_VERSION }}"

Example config/main.yml entry:

compose:
  services:
    myapp:
      image:   myapp/myapp
      version: "1.0"

Example vars/main.yml entry:

APP_IMAGE:   "{{ lookup('config', application_id, 'compose.services.myapp.image') }}"
APP_VERSION: "{{ lookup('config', application_id, 'compose.services.myapp.version') }}"

When Dockerfile.j2 is acceptable

A templates/Dockerfile.j2 is acceptable when the file contains Jinja2 control-flow logic that cannot be expressed through Docker ARG alone — for example, a conditional build step that installs an optional component only when a feature flag is enabled.

Even in that case, you SHOULD minimize the templated surface: use {{ variables }} only where necessary and keep the static parts of the Dockerfile readable without rendering it.

Lint

The repository lint suite checks for templates/Dockerfile.j2 files automatically:

• A Dockerfile.j2 with no Jinja2 control-flow logic causes a test failure. It MUST be migrated to files/Dockerfile with ARG declarations.
• A Dockerfile.j2 with Jinja2 control-flow logic emits a warning only. The warning signals that the file should be reviewed to see whether the logic can be eliminated and the file migrated.

See test_dockerfile_templates.py for the implementation.