Dependencies issue in recent Docker images

[babolivier]

CI complains about a missing on matrix-common and we've seen similar reports about Jinja:

Traceback (most recent call last):

  File "/start.py", line 11, in <module>

    import jinja2

ModuleNotFoundError: No module named 'jinja2'

[babolivier]

Possibly fixed by #12853

[erikjohnston]

What was happening was that poetry.core package got updated, which combined with our random pinned poetry version caused it to think that all deps were dev dependencies, and so poetry export would write an empty requirements.txt and so no deps were actually installed in the final image.