Update Dockerfile to add ffmpeg via apt packages

RUN chmod +x /custom_entrypoint.sh

RUN mkdir -p /var/www/skeleton/Documents && mkdir -p /var/www/skeleton/Images

# Install unzip for unzipping artifacts

RUN apt-get update && apt-get install unzip

# Install ffmpeg for video preview generation 

RUN apt-get update && apt-get install unzip ffmpeg

RUN curl -fsSL -o notes.tar.gz \

    "https://github.com/nextcloud/notes/releases/download/v${NOTES_VERSION}/notes.tar.gz" && \