chore(build): Address warnings in the docker/django/Dockerfile

release, the action began reporting build configuration warnings.
Neato! It highlights three issues, each addressed in this patch.

```
 3 warnings found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 1)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 44)
 - UndefinedVar: Usage of undefined variable '$PYTHONPATH' (line 75)
```

docker/django/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.13-slim as build-base
+FROM python:3.13-slim AS build-base
 
 RUN apt-get update --option "Acquire::Retries=3" --quiet=2 && \
     apt-get install \
@@ -41,7 +41,7 @@ RUN python -m venv $POETRY_HOME && \
     ln -s $POETRY_HOME/bin/poetry "$(dirname $(which python))/poetry"  # make accessible via $PATH
 
 ARG BUILD_ENV=prod
-FROM build-base as python-base
+FROM build-base AS python-base
 
 WORKDIR $PYSETUP_PATH
 
@@ -71,7 +71,7 @@ RUN mkdir /var/log/courtlistener \
 WORKDIR /opt/courtlistener
 
 
-ENV PYTHONPATH="${PYTHONPATH}:/opt/courtlistener"
+ENV PYTHONPATH="/opt/courtlistener"
 
 USER root
 ## Creates a directory for Hyperscan db