Add ENTRYPOINT, bump Python version in Dockerfile

* Dockerfile improvements: Add ENTRYPOINT, bump Python version (+build arg).

* Dockerfile: Add `syntax` version and OCI 'source' Annotation (`LABEL`).

Dockerfile

@@ -1,2 +1,12 @@
-FROM python:3.9-slim
-RUN pip3 install cloudinary-cli --no-cache
+# syntax=docker/dockerfile:1
+
+ARG PYTHON_VERSION
+FROM python:${PYTHON_VERSION:-3.12-slim}
+
+# For available labels, see OCI Annotations Spec docs:
+# https://specs.opencontainers.org/image-spec/annotations/#pre-defined-annotation-keys
+LABEL org.opencontainers.image.source="https://github.com/cloudinary/cloudinary-cli"
+
+RUN pip3 install --no-cache cloudinary-cli
+
+ENTRYPOINT [ "cloudinary" ]