fix pixiv tests, install ffmpeg

config/docker/danbooru-ci

@@ -3,6 +3,7 @@
 FROM r888888888/danbooru-base
 
 USER root
+RUN apt-get install -y ffmpeg
 COPY . /app
 RUN chown -R danbooru:danbooru .
 USER danbooru

test/unit/sources/pixiv_test.rb

@@ -1,10 +1,12 @@
 require 'test_helper'
+require 'resolv-replace'
 
 module Sources
   class PixivTest < ActiveSupport::TestCase
     def get_source(source)
       @site = Sources::Site.new(source)
       @site.get
+      @site.strategy.agent.shutdown
       @site
     end