- Fix Drush installation

- Consistent naming of arguments
- Add deprecated install note

workspace/Dockerfile

@@ -299,13 +299,17 @@ RUN if [ ${INSTALL_SWOOLE} = true ]; then \
 # Drush:
 ###########################################################################
 
+# Deprecated install of Drush 8 and earlier versions.
+# Drush 9 and up require Drush to be listed as a composer dependency of your site.
+
 USER root
 
 ARG INSTALL_DRUSH=false
+ARG DRUSH_VERSION
+ENV DRUSH_VERSION ${DRUSH_VERSION}
 
 RUN if [ ${INSTALL_DRUSH} = true ]; then \
     apt-get -y install mysql-client && \
-    # Install Drush 8 with the phar file.
     curl -fsSL -o /usr/local/bin/drush https://github.com/drush-ops/drush/releases/download/${DRUSH_VERSION}/drush.phar | bash && \
     chmod +x /usr/local/bin/drush && \
     drush core-status \