added support for DISCOURSE_ADDITIONAL_PLUGINS build arg

Dockerfile

@@ -11,7 +11,7 @@ ENV DISCOURSE_VERSION=1.7.0.beta3 \
     DISCOURSE_SERVE_STATIC_ASSETS=true \
     GIFSICLE_VERSION=1.87 \
     PNGQUANT_VERSION=2.4.1
-
+    
 RUN curl --silent --location https://deb.nodesource.com/setup_4.x | bash - \
  && apt-get update && apt-get install -y --no-install-recommends \
       autoconf \
@@ -46,6 +46,22 @@ RUN git clone --branch v${DISCOURSE_VERSION} https://github.com/discourse/discou
  && git remote set-branches --add origin tests-passed \
  && bundle config build.nokogiri --use-system-libraries \
  && bundle install --deployment --without test --without development
+ 
+
+# install discourse plugins
+# assumptions: no spaces in URLs (urlencoding is a thing)
+# 
+# this expects a git-cloneable link
+ARG DISCOURSE_ADDITIONAL_PLUGINS=
+RUN if [ "$DISCOURSE_ADDITIONAL_PLUGINS" != "" ]; then \
+        cd plugins/ \
+        for PACKAGE_LINK in $DISCOURSE_ADDITIONAL_PLUGINS; do \
+            git clone "$PACKAGE_LINK"; \
+        done; \
+        bundle exec rake plugin:update plugin=discourse-plugin-checklist; \
+        bundle exec rake assets:clean; \
+        bundle exec rake assets:precompile; \
+    fi
 
 EXPOSE 3000
 CMD ["bundle", "exec", "rails", "server", "-b", "0.0.0.0"]