overcomplicated method of downloading .deb for signal-desktop

getsignaldeb.sh

#!/bin/bash -xe
# overcomplicated method of downloading .deb for signal-desktop
docker rmi -f getsignal
cat <<EOF | docker build -t getsignal  -f- .
FROM ubuntu
RUN apt update
RUN apt install -y wget sudo gnupg
RUN wget -O- https://updates.signal.org/desktop/apt/keys.asc | apt-key add -
RUN echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" >> /etc/apt/sources.list.d/signal-xenial.list
RUN apt update
WORKDIR /out
ENTRYPOINT ["/usr/bin/apt", "download", "signal-desktop"]
EOF
docker run -v ${PWD}:/out -it --rm getsignal