misc: Simplify backend environment configuration in Docker image

This change moves the virtualenv creation in the `Dockerfile` to a
separate stage, to simplify isolating the process and reduce the need
for uninstalling build dependencies.

The approach is similar to the one explained in [1]. It relies on
building a virtualenv folder, and copying it in the final stage.
Changing the `PATH` environment variable makes the virtualenv usable by
default, without affecting the default Python installation.

Also, added Dockerfile arguments for Alpine, nginx, and Python versions,
as some of them are reused, and also simplifies testing new versions.

An extra side effect is that the image size for the final stage is
reduced from 315MB to 262MB.

[1] https://scribe.rip/@albertazzir/blazing-fast-python-docker-builds-with-poetry-a78a66f5aed0

docker/init_scripts/init

@@ -5,10 +5,6 @@ set -o nounset           # treat unset variables as an error
 set -o pipefail          # treat errors in pipes as fatal
 shopt -s inherit_errexit # inherit errexit
 
-# use virtualenvs
-# shellcheck disable=SC1091
-source /backend/bin/activate
-
 # make it possible to disable the inotify watcher process
 ENABLE_RESCAN_ON_FILESYSTEM_CHANGE="${ENABLE_RESCAN_ON_FILESYSTEM_CHANGE:="true"}"