Update linux installation instructions for FFmpeg v4.1

Author: scriptorian

.circleci/images/testbeam10-4.1/Dockerfile

@@ -4,6 +4,7 @@ FROM circleci/node:10
 RUN sudo apt-get update \
   && sudo apt-get install software-properties-common \
   && sudo add-apt-repository ppa:jonathonf/ffmpeg-4 \
+  && sudo apt-get update \
   && sudo apt-get install libavcodec-dev libavformat-dev libavdevice-dev libavfilter-dev libavutil-dev libpostproc-dev libswresample-dev libswscale-dev
 
 # delete all the apt list files since they're big and get stale quickly

README.md

@@ -98,6 +98,7 @@ To ensure that sufficient threads are available to process several requests in p
 On Linux, use the appropriate package manager to install the FFmpeg 4.1 development dependencies first. An error will be printed if these cannot be found at expected locations. For example, on Ubuntu:
 
     sudo add-apt-repository ppa:jonathonf/ffmpeg-4
+    sudo apt-get update
     sudo apt-get install libavcodec-dev libavformat-dev libavdevice-dev libavfilter-dev libavutil-dev libpostproc-dev libswresample-dev libswscale-dev
 
 An example for Debian / Redhat / Fedora / CentOs will follow.