One layer for env

Dockerfile

@@ -2,7 +2,12 @@ FROM rails:4.1
 
 WORKDIR /usr/src/app
 
-ENV DISCOURSE_VERSION 1.4.7
+ENV DISCOURSE_VERSION=1.4.7 \
+    RAILS_ENV=production \
+    RUBY_GC_MALLOC_LIMIT=90000000 \
+    DISCOURSE_DB_HOST=postgres \
+    DISCOURSE_REDIS_HOST=redis \
+    DISCOURSE_SERVE_STATIC_ASSETS=true
 
 RUN apt-get update && apt-get install -y --no-install-recommends imagemagick libxml2 \
  && rm -rf /var/lib/apt/lists/*
@@ -12,11 +17,5 @@ RUN curl -L https://github.com/discourse/discourse/archive/v${DISCOURSE_VERSION}
  && bundle config build.nokogiri --use-system-libraries \
  && bundle install --deployment --without test --without development
 
-ENV RAILS_ENV production
-ENV RUBY_GC_MALLOC_LIMIT 90000000
-ENV DISCOURSE_DB_HOST postgres
-ENV DISCOURSE_REDIS_HOST redis
-ENV DISCOURSE_SERVE_STATIC_ASSETS true
-
 EXPOSE 3000
 CMD ["rails", "server"]