Update Dockerfile to use Python 3.9

[StefanMallia]

Description

Small change to the Dockerfile to use Python 3.9 as required by pyproject.toml. Currently the docker build fails on "Package 'pgcli' requires a different Python: 3.8.20 not in '>=3.9'.

[dbaty]

Python 3.9 has reached its end of life last October (see here). We should rather use the latest version: 3.14.

[StefanMallia]

True, but this is just to provide a simple fix for the Docker build.

[dbaty]

Using the latest version of Python should not be notably more complex than your fix, is it? :)