Change ARG location to hopefully resolve empty string issue

pica-etherpad/Dockerfile

@@ -3,9 +3,12 @@
 FROM node:10-buster-slim as base
 LABEL maintainer="quentinduchemin@tuta.io"
 
+ENV NODE_ENV=production
+
+FROM base as downloader
+
 ARG ETHERPAD_VERSION_BUILD=1.8.0
 ARG ETHERPAD_LANDING_PAGE_VERSION_BUILD=v0.1
-ARG NODE_ENV_BUILD=production
 ARG ETHERPAD_PLUGINS="\
   ep_align ep_author_hover ep_chatdate ep_colibris_full_width \
   ep_comments_page ep_countable ep_delete_empty_pads ep_font_color \
@@ -13,9 +16,6 @@ ARG ETHERPAD_PLUGINS="\
   ep_pads_stats ep_page_view ep_prompt_for_name ep_set_title_on_pad \
   ep_subscript_and_superscript ep_tables2 ep_user_font_size"
 
-ENV NODE_ENV=${NODE_ENV_BUILD}
-
-FROM base as downloader
 RUN apt-get update && \
     apt-get install -y \
       git \