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Hi,
There seems no doc or example about how to customize the Dockerfile
with template override.
For example, if I want to run ./tools/build.py --template-only
--template-override ./heat-extend.j2 heat, what is the correct format
for heat-extend.j2?

When the content of heat-extend.j2 is

{% set heat_base_packages = heat_base_packages + ['vim', 'emacs'] %}

The above command gives the error "jinja2.exceptions.UndefinedError:
'heat_base_packages' is undefined".
Thanks in advance.

- Hui