refactor: abstract out the python image tag (#18109)

Author: miketheman

Dockerfile

@@ -1,3 +1,6 @@
+# Set variables reused in Dockerfile
+ARG PYTHON_IMAGE_VERSION=3.13.2-slim-bookworm
+
 # First things first, we build an image which is where we're going to compile
 # our static assets with. We use this stage in development.
 FROM node:24.0.0-bookworm AS static-deps
@@ -38,7 +41,7 @@ RUN NODE_ENV=production npm run build
 
 
 # We'll build a light-weight layer along the way with just docs stuff
-FROM python:3.13.2-slim-bookworm AS docs
+FROM python:${PYTHON_IMAGE_VERSION} AS docs
 
 # By default, Docker has special steps to avoid keeping APT caches in the layers, which
 # is good, but in our case, we're going to mount a special cache volume (kept between
@@ -105,7 +108,7 @@ USER docs
 
 # Now we're going to build our actual application, but not the actual production
 # image that it gets deployed into.
-FROM python:3.13.2-slim-bookworm AS build
+FROM python:${PYTHON_IMAGE_VERSION} AS build
 
 # Define whether we're building a production or a development image. This will
 # generally be used to control whether or not we install our development and
@@ -187,7 +190,7 @@ RUN --mount=type=cache,target=/root/.cache/pip \
 
 # Now we're going to build our actual application image, which will eventually
 # pull in the static files that were built above.
-FROM python:3.13.2-slim-bookworm
+FROM python:${PYTHON_IMAGE_VERSION}
 
 # Setup some basic environment variables that are ~never going to change.
 ENV PYTHONUNBUFFERED 1