shizheiyang/ffmpeg repository overview

docker pull shizheiyang/ffmpeg
docker tag shizheiyang/ffmpeg ffmpeg

docker run --rm -v /tmp/mp3:/opt/mp3 ffmpeg -i /opt/mp3/test.m4a /opt/mp3/test.mp3

SELinux enabled:
docker run --rm -v /tmp/mp3:/opt/mp3:Z ffmpeg -i /opt/mp3/test.m4a /opt/mp3/test.mp3

Or

docker run --rm -v /tmp/file.m4a:/tmp/file.m4a ffmpeg -i /tmp/file.m4a -acodec libmp3lame -f mp3 - > /tmp/file.mp3