Update doorkeeper dependency and replace libav with backported ffmpeg in docker

Dockerfile

@@ -2,7 +2,8 @@ FROM ruby:2.2.4
 
 ENV RAILS_ENV=production
 
-RUN apt-get update -qq && apt-get install -y build-essential libpq-dev libxml2-dev libxslt1-dev nodejs nodejs-legacy npm libav-tools && rm -rf /var/lib/apt/lists/*
+RUN echo 'deb http://httpredir.debian.org/debian jessie-backports main contrib non-free' >> /etc/apt/sources.list
+RUN apt-get update -qq && apt-get install -y build-essential libpq-dev libxml2-dev libxslt1-dev nodejs nodejs-legacy npm ffpmeg && rm -rf /var/lib/apt/lists/*
 RUN mkdir /mastodon
 
 WORKDIR /mastodon

Gemfile.lock

@@ -101,7 +101,7 @@ GEM
     docile (1.1.5)
     domain_name (0.5.20160615)
       unf (>= 0.0.5, < 1.0.0)
-    doorkeeper (4.1.0)
+    doorkeeper (4.2.0)
       railties (>= 4.2)
     dotenv (2.1.1)
     dotenv-rails (2.1.1)