Disable concurrent when precompile the assets

Concurrency are raising failure during the precompile.
There is an issue on sprocket related to this issue
rails/sprockets#581.

src/api/config/initializers/assets.rb

@@ -9,3 +9,8 @@
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.
 Rails.application.config.assets.precompile += ['webui2/webui2.css', 'webui2/application.js', 'webui/obs_factory/application.css', 'webui2/cm2/*.js']
+
+# FIXME: concurrency are raising failure during the precompile.
+# For more information https://github.com/rails/sprockets/issues/581
+# and https://github.com/sass/sassc-ruby/pull/138
+Rails.application.config.assets.export_concurrent = false

src/api/docker-files/Dockerfile.minitest

@@ -0,0 +1,29 @@
+# This is just a thin layer on top of the frontend-backend container that makes
+# sure different users can run it without the contained rails app generating
+# files in the git checkout with some strange user...
+
+FROM registry.opensuse.org/obs/server/unstable/container/leap151/containers/openbuildservice/frontend-backend
+
+# Configure our user
+ARG CONTAINER_USERID
+RUN usermod -u 1000 frontend
+
+# We copy the Gemfiles into this intermediate build stage so it's checksum
+# changes and all the subsequent stages (a.k.a. the bundle install call below)
+# have to be rebuild. Otherwise, after the first build of this image,
+# docker would use it's cache for this and the following stages.
+ADD Gemfile /obs/src/api/Gemfile
+ADD Gemfile.lock /obs/src/api/Gemfile.lock
+RUN chown -R frontend /obs/src/api
+
+# Now do the rest as the user with the same ID as the user who
+# builds this container
+USER frontend
+WORKDIR /obs/src/api
+
+# Refresh our bundle
+RUN export NOKOGIRI_USE_SYSTEM_LIBRARIES=1; bundle install --jobs=3 --retry=3 || bundle install --jobs=3 --retry=3
+
+CMD ["/bin/bash", "-l"]
+
+