Install latest Poetry in container

[leplatrem]

This removes the warning about old version being installed.

It has the downside of potentially not being fully reproducible, but I think that this is acceptable if we have #102

[leplatrem]

I also ran into this:

[2/3] STEP 8/8: RUN poetry install --no-dev --no-root
Traceback (most recent call last):
  File "/opt/poetry/bin/poetry", line 17, in <module>
    from poetry.console import main
  File "/opt/poetry/lib/poetry/console/__init__.py", line 1, in <module>
    from .application import Application
  File "/opt/poetry/lib/poetry/console/application.py", line 3, in <module>
    from cleo import Application as BaseApplication
ModuleNotFoundError: No module named 'cleo'

[grahamalama]

@leplatrem I just noticed this in the docs -- looks like a new method to install poetry. Just got a working build with it.

curl -sSL https://install.python-poetry.org | python3 -

[leplatrem]

we could still pin the Poetry version using the env var like before.

[grahamalama]

I'm curious why this PR triggered a cloudops "deployment". I don't think it deployed anything -- just built the container as a test?