Use ARG instead of ENV for build time only environment variable

Dockerfile

@@ -5,7 +5,7 @@ FROM alpine:${ALPINE_VERSION}
 RUN apk add --no-cache git python3 python3-dev py3-pip py3-setuptools build-base
 
 # build wheels in a build stage
-ENV VIRTUAL_ENV=/opt/venv
+ARG VIRTUAL_ENV=/opt/venv
 ENV PATH=${VIRTUAL_ENV}/bin:${PATH}
 
 RUN python3 -m venv ${VIRTUAL_ENV}
@@ -23,7 +23,7 @@ FROM alpine:${ALPINE_VERSION}
 # install python, git, bash, mercurial
 RUN apk add --no-cache git git-lfs python3 py3-pip py3-setuptools bash docker mercurial
 
-ENV VIRTUAL_ENV=/opt/venv
+ARG VIRTUAL_ENV=/opt/venv
 ENV PATH=${VIRTUAL_ENV}/bin:${PATH}
 
 RUN python3 -m venv ${VIRTUAL_ENV}