feat: configurable basepath

[ItzDerock]

Fixes #102

Adds a configurable environment variable BASEPATH that can be set.

docker run --rm -p 8080:80 -e BASEPATH="/chatpadai" chatpadai

Note that the docker container will still serve the files at the root, you will need to configure another reverse-proxy. This just makes the app aware of the basepath so it can properly fetch the config and other assets.