Introduce Python v3.12 image (#8)

Use rc (unstable Python version) image until it’s released as stable

Install gcc and libffi-dev required to install poetry since some C/C++ packages do not already provide pre-build wheels for Py3.12

Tag 1.4.0 was pushed for this change. The tag 1.5.0 will be pushed once Python 3.12 is released, and the check boxes in #8 (closed) are fix.