build: build and publish image nightly

Removes need to update toolchain for every action step

Dockerfile

@@ -1,5 +1,7 @@
 FROM docker.io/rust:1.84.0-slim-bookworm
 
+LABEL org.opencontainers.image.source=https://git.kemitix.net/kemitix/rust
+
 # nodejs - runtime used by forgejo/github actions
 # curl - to download cargo-binstall
 # clang-16 & mold - faster linkers for rust
@@ -25,6 +27,13 @@ RUN cargo binstall -y \
   cargo-mutants@25.0 \
   release-plz@0.3
 
+COPY update.sh /
+
+# should be a no-op if the FROM line is up-to-date
+RUN /update.sh stable 
+
+RUN /update.sh nightly
+
 COPY entrypoint.sh /
 
 RUN git config --global user.email "action@git.kemitix.net" && \