style: upgrade, not dist-upgrade

We want security updates, not to change major ver, so that's fine.

Dockerfile

@@ -3,7 +3,7 @@ FROM ruby:2.7.5-slim-bullseye
 RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get \
         -o Dpkg::Options::="--force-confnew" --allow-remove-essential --allow-change-held-packages -fuy \
-        dist-upgrade \
+        upgrade \
     && apt-get install -y --no-install-recommends \
         cmake \
         pkg-config \