Add debian packaging files

This doesn't quite work, since the debuild stuff refuses to see the
rustup-installed cargo/rustc, which is required for our build.

Dockerfile

@@ -0,0 +1,30 @@
+FROM debian:bullseye-slim
+
+ARG RUST_VERSION=1.60
+
+WORKDIR /bscreensaver
+
+RUN apt-get update && \
+    apt-get -y full-upgrade && \
+    apt-get -y install curl devscripts && \
+    mkdir -p /build
+
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain nightly
+
+# Do this first to get our build deps cached.
+COPY debian ./debian
+RUN apt-get build-dep -y .
+
+# Copy only what we need, to avoid bloating the image with e.g. the target/ dir
+COPY async-xcb ./async-xcb
+COPY command ./command
+COPY dbus-service ./dbus-service
+COPY dialog-gtk3 ./dialog-gtk3
+COPY locker ./locker
+COPY settings ./settings/
+COPY systemd ./systemd
+COPY util ./util
+COPY xcb-xembed ./xcb-xembed
+COPY bscreensaver.toml.example Cargo.* Makefile ./
+
+CMD bash