fix(ci): stick to Debian Bullseye until we are ready to move on

Dockerfile

@@ -18,7 +18,7 @@ RUN npm ci
 COPY assets /assets
 RUN /node_modules/.bin/webpack-cli
 
-FROM python:3-slim
+FROM python:3-slim-bullseye
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \
       zlib1g libfreetype6 libjpeg62-turbo libopenjp2-7 libtiff5 liblcms2-2 \