library: Dockerfile fails loudly and verifies the image

A '|| true' at the end of the install chain swallowed any earlier failure, so a
transient/blocked apt step produced a successful build with no Koha in it (found
when an update rebuilt without the proxy CA). Steps are now separate, memcached and
cron are installed explicitly, and a final step refuses to produce an image that is
missing koha-create, memcached, cron, apache2ctl, zebrasrv, ... Quick Add's image
verifies yaz and python3 the same way.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZSot4XbUViwRFgLpRf

library/quickadd/Dockerfile

@@ -13,6 +13,8 @@ RUN apt-get update \
 COPY certs/ /usr/local/share/ca-certificates/extra/
 RUN update-ca-certificates
 
+RUN command -v yaz-client >/dev/null && command -v yaz-marcdump >/dev/null && command -v python3 >/dev/null
+
 RUN useradd --system --uid 10001 --home-dir /app quickadd \
  && mkdir -p /app /data && chown quickadd /data
 COPY app.py index.html /app/