DX-1846: Add x86_64-linux platform to lockfile

Author: asbjornu

.docker/Dockerfile

@@ -62,6 +62,7 @@ RUN bundle config build --use-system-libraries && \
     bundle config build.nokogiri --use-system-libraries && \
     bundle config set system 'true' && \
     bundle config set clean 'true' && \
+    bundle lock --add-platform x86_64-linux && \
     bundle install \
         --gemfile="${JEKYLL_VAR_DIR}/entrypoint/Gemfile" \
         --jobs=4