Add libjsonnet++ for publishing-api Content Schemas.

Only publishing-api strictly needs this right now, but it's a small
library so adding it here is reasonable. Saves having to pull down the
Apt catalog in the pubapi build, and reduces the chance of
copypasta/cargo-culting extra steps from the pubapi Dockerfile.

See alphagov/publishing-api#2788.

base.Dockerfile

@@ -118,7 +118,8 @@ ENV PATH=${TMPDIR_FOR_RUBY_WRAPPERS_DIR}:${PATH}
 # Install node.js, yarn and other runtime dependencies.
 COPY --from=builder /usr/share/keyrings/nodesource.gpg /usr/share/keyrings/
 
-RUN install_packages ca-certificates curl libjemalloc-dev libgdbm6 libyaml-0-2 \
+RUN install_packages ca-certificates curl libjemalloc-dev libgdbm6 \
+      libjsonnet0 libyaml-0-2 \
       libmariadb3 libpq5 mariadb-client postgresql-client tzdata; \
       curl -fsSL https://deb.nodesource.com/setup_18.x | bash; \
     install_packages nodejs; \