ENH: Allow specifying PYTHON_VERSION in Dockerfile

Dockerfile

@@ -1,12 +1,13 @@
 ARG GDAL=ubuntu-small-3.3.3
 FROM osgeo/gdal:${GDAL} AS gdal 
 ENV LANG="C.UTF-8" LC_ALL="C.UTF-8"
+ARG PYTHON_VERSION=3.8
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-    python3 python3-pip python3-dev python3-venv cython3 g++ && \
+    python${PYTHON_VERSION} python3-pip python${PYTHON_VERSION}-dev python${PYTHON_VERSION}-venv cython3 g++ && \
     rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY requirements*.txt ./
-RUN python3 -m venv /venv && \
+RUN python${PYTHON_VERSION} -m venv /venv && \
     /venv/bin/python -m pip install -U pip && \
     /venv/bin/python -m pip install -r requirements-dev.txt