Yup, I came to the same conclusion. Added the following to Dockerfile-prerelease and the rest worked:

# Install build dependencies for FFmpeg

RUN apk add --no-cache --virtual .build-deps \

build-base coreutils git nasm pkgconf yasm \

rtmpdump-dev openssl-dev x264-dev x265-dev freetype-dev libvorbis-dev opus-dev libvpx-dev lame-dev libtheora-dev xvidcore-dev \

fontconfig-dev fribidi-dev libass-dev

# Clone and build FFmpeg with librtmp disabled

RUN git clone --depth 1 https://git.ffmpeg.org/ffmpeg.git /tmp/ffmpeg && \

cd /tmp/ffmpeg && \

./configure \

--enable-gpl \

--enable-nonfree \

--enable-openssl \

--enable-libx264 \

--enable-libx265 \

--enable-libfreetype \

--enable-libvorbis \

--enable-libopus \

--enable-libvpx \

--enable-libmp3lame \

--enable-libtheora \

--enable-libxvid \

--enable-libfontconfig \

--enable-libfribidi \

--enable-libass \

--disable-debug \

--disable-doc \

--disable-ffplay && \

make -j$(nproc) && \

make install && \

cd /tmp && \

rm -rf /tmp/ffmpeg