Build info for CircleCI test image

Author: scriptorian

.circleci/test_image/Dockerfile

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+FROM cimg/node:16.13
+
+# install FFmpeg
+RUN sudo apt-get update \
+  && sudo apt-get install software-properties-common \
+  && sudo add-apt-repository ppa:savoury1/ffmpeg4 \
+  && sudo apt-get update \
+  && sudo apt-get upgrade && sudo apt-get dist-upgrade \
+  && 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
+RUN sudo rm -rf /var/lib/apt/lists/*
+# this forces "apt-get update" in dependent images, which is also good

.circleci/test_image/build.md

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+# Instructions for building the CircleCI docker image for testing
+
+- install docker desktop
+- cd to this directory
+- docker build -t streampunkmedia/testbeam:x-y.z .
+- run container locally to check build
+- push to Docker Hub
+- update config.yml to pull new version tag
+- push to git to trigger new build and test
+
+(x: NodeAPI base version, y.z: FFmpeg build number)
+
+See https://circleci.com/developer/images/image/cimg/node for CircleCI docker image tags