Make debian package building work

I gave up on getting it to build on bullseye, as its cargo/rustc is too
old for the 2021 edition.  Building newer deb packages of rust is
apparently a difficult, manual process.  I tried installing rustup and
pulling the latest compiler, but that failed with strange errors that I
am tired of debugging.

Dockerfile

@@ -1,15 +1,11 @@
-FROM debian:bullseye-slim
-
-ARG RUST_VERSION=1.60
-
-WORKDIR /bscreensaver
+FROM debian:bookworm-slim
 
 RUN apt-get update && \
     apt-get -y full-upgrade && \
-    apt-get -y install curl devscripts && \
-    mkdir -p /build
+    apt-get -y install devscripts && \
+    mkdir -p /bscreensaver-build
 
-RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+WORKDIR /bscreensaver
 
 # Do this first to get our build deps cached.
 COPY debian ./debian