I expect this to work, but it doesn't

Dockerfile

FROM varnish

RUN apt update -y && apt install -y build-essential automake curl libtool git
RUN curl -s https://packagecloud.io/install/repositories/varnishcache/varnish60/script.deb.sh
RUN apt install -y varnish-modules

FROM debian:bullseye

RUN apt update -y && apt install -y curl build-essential dpkg-dev
RUN curl -s https://packagecloud.io/install/repositories/varnishcache/varnish70/script.deb.sh | bash
RUN apt update -y && apt install -y varnish varnish-dev

RUN apt update -y && apt install -y build-essential automake libtool curl git python3-docutils
RUN git clone https://github.com/varnish/varnish-modules.git /tmp/vm && \
    cd /tmp/vm && \
    ./bootstrap && \
    ./configure && \
    make && \
    make check && \
    make install

This ended up working. The version of varnish-dev from packagecloud must match the version of varnish from the primary debian sources.