Can't install Poetry in Ubuntu 20.04 Docker image due to setuptools error

I'm trying to build the following Dockerfile:

FROM ubuntu:20.04

RUN apt-get update && \
    apt-get install -y curl software-properties-common && \
    add-apt-repository ppa:deadsnakes/ppa && \
    apt install -y python3.11 python3.11-distutils && \
    curl -sS https://bootstrap.pypa.io/get-pip.py | python3.11 && \
    python3.11 -m pip install poetry==1.2.2

However, it fails to install poetry with the following error message:

Collecting msgpack>=0.5.2
  Downloading msgpack-1.0.4.tar.gz (128 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 128.1/128.1 kB 15.2 MB/s eta 0:00:00
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
ERROR: Exception:
Traceback (most recent call last):
...
  File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1142, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'setuptools'

If I replace the install command for Poetry at the end and replace it with python3.11 -m pip list then the output clearly shows setuptools being installed:

Package             Version
------------------- -----------
...
setuptools          67.2.0
...

I can also import setuptools without problems when I open a Python interpreter. What is going on?

Note that I'm unable to use a newer Ubuntu version or pythonX.Y image due to other requirements.

Don't install poetry into the global Python environment (nor into your project venv), or it'll influence package compatibility within your project/system dependencies.

See poetry docs on how to give poetry its own venv when doing a "manual install". Otherwise, use the official poetry install script which does that for you.