Use python 3.8 to install Django 4

[Xpirix]

According to #56 and #61 , we should use Django 4 to fix the spatial filter map. I think this is also an opportunity to upgrade to the new LTS because version 3.2 extended support will end in April 2024.

Current situation

• This version of Django needs Python 3.8+.
• In production, the build raises an error when using the Python 3.8 image:

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <the_pubkey> 

Proposed fixes

• Upgrade to Python 3.8 and Django 4. I pushed the image on the Docker hub (xpirix/qgisfeed:production-1.4) so we don't have to upgrade the Docker version or build it on the server.
• Upgrade to Postgresql 16 because Django 4 depends on 12+ (The current version is 11).

EDIT: I also added the code folder in the mounted volume so we don't have to build the image to have each change inside the container.