Clarify pip cache ARG choice comment

Co-authored-by: Erik Sundell <[email protected]>

Author: minrk

images/hub/Dockerfile

@@ -53,8 +53,8 @@ RUN apt-get update && \
         tini \
  && rm -rf /var/lib/apt/lists/*
 
-# set env for pip cache,
-# but use ARG to avoid persisting in image
+# set pip's cache directory using this environment variable, and use
+# ARG instead of ENV to ensure its only set when the image is built
 ARG PIP_CACHE_DIR=/tmp/pip-cache
 
 # install wheels built in the build-stage

images/singleuser-sample/Dockerfile

@@ -7,8 +7,8 @@ FROM python:3.9-bullseye as build-stage
 
 WORKDIR /build-stage
 
-# set env for pip cache,
-# but use ARG to avoid persisting in image
+# set pip's cache directory using this environment variable, and use
+# ARG instead of ENV to ensure its only set when the image is built
 ARG PIP_CACHE_DIR=/tmp/pip-cache
 
 # These are mounted into the final image for installation