Fix export-server render 500: stop purging Chrome's wget dependency

The render test (#675) returned HTTP 500 on every diagram. Captured logs
show Puppeteer failing with "Tried to find the browser at the configured
path (/usr/bin/google-chrome-stable), but no executable was found".

google-chrome-stable Depends on wget, so the `apt-get remove -y --purge
git wget` cleanup uninstalls Chrome along with wget, leaving
PUPPETEER_EXECUTABLE_PATH (set in dc57dcc) pointing at a path that no
longer exists. This is why the prior fix was necessary but not sufficient.

Only purge git (a build-time clone tool, not a Chrome dependency); keep
wget. Add a `test -x /usr/bin/google-chrome-stable` build-time guard so a
future regression fails the build immediately rather than at render time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: davidjgraph

image-export/Dockerfile

@@ -27,7 +27,12 @@ RUN apt-get update \
     && cd /home/pptruser \
     && git clone https://github.com/jgraph/draw-image-export2.git \
     && chown -R pptruser:pptruser /home/pptruser \
-    && apt-get remove -y --purge git wget \
+    # Only purge git (a build-time clone tool). Do NOT purge wget: it is a
+    # runtime dependency of google-chrome-stable, so removing it uninstalls
+    # Chrome and leaves PUPPETEER_EXECUTABLE_PATH dangling -> HTTP 500 on render.
+    && apt-get remove -y --purge git \
+    # Fail the build early if the cleanup ever removes Chrome again.
+    && test -x /usr/bin/google-chrome-stable \
     && rm -rf /var/lib/apt/lists/*
 
 USER pptruser