Only set env vars near the end

Improves the reusability of layers as it adds those env vars
to each RUN command, so they look unique, even though the
run commands are not using the env vars.

Dockerfile

@@ -67,12 +67,6 @@ EOF
 
 FROM docker.io/python:${PYTHON_VERSION}-slim-bookworm as runner
 
-ARG KARROT_VERSION="unknown"
-ENV KARROT_VERSION="${KARROT_VERSION}"
-
-ARG KARROT_COMMIT="unknown"
-ENV KARROT_COMMIT="${KARROT_COMMIT}"
-
 ENV PYTHONUNBUFFERED=1
 
 # Only dependencies needed for runtime
@@ -123,4 +117,13 @@ USER $USERNAME
 
 COPY docker/entrypoint.sh /entrypoint.sh
 
+# Keep these near the end, as they are added to any RUN commands
+# which means new layers are created when just the version/commit
+# has changed
+ARG KARROT_VERSION="unknown"
+ENV KARROT_VERSION="${KARROT_VERSION}"
+
+ARG KARROT_COMMIT="unknown"
+ENV KARROT_COMMIT="${KARROT_COMMIT}"
+
 ENTRYPOINT ["/entrypoint.sh"]