Allow the apt-get steps to be cached when rebuilding with different…

… `octoprint_ref` build argument.

minimal/Dockerfile

@@ -2,9 +2,6 @@ ARG PYTHON_BASE_IMAGE=3-slim-buster
 
 FROM python:${PYTHON_BASE_IMAGE} AS build
 
-ARG octoprint_ref 
-ENV octoprint_ref ${octoprint_ref:-master}
-
 RUN apt-get update && apt-get install -y \
   build-essential \
   curl \
@@ -26,6 +23,9 @@ ENV PYTHONUSERBASE /octoprint/plugins
 ENV PIP_USER true
 ENV PATH "${PYTHONUSERBASE}/bin:${PATH}"
 
+ARG octoprint_ref 
+ENV octoprint_ref ${octoprint_ref:-master}
+
 RUN	mkdir tmp && curl -fsSLO --compressed --retry 3 --retry-delay 10 \
   https://github.com/OctoPrint/OctoPrint/archive/${octoprint_ref}.tar.gz \
   && tar xzf ${octoprint_ref}.tar.gz --strip-components 1 -C tmp \