Dump export-server logs after render in CI

The render test asserts a 200 + PNG, but when Puppeteer/Chrome throws the
worker returns a generic 500 "Error!" and logs the real exception via
winston. The existing 'docker logs' runs before the render request, so it
never captures that exception. Dump the container logs (and response body)
after the render request so a failing render surfaces the actual error and
stack trace in the CI output.

Diagnostic for the render 500 in run #675.

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

Author: davidjgraph

.github/workflows/docker-image-export.yml

@@ -32,6 +32,14 @@ jobs:
           --data-urlencode 'scale=1' \
           --data-urlencode 'xml=<mxGraphModel><root><mxCell id="0"/><mxCell id="1" parent="0"/><mxCell id="2" value="CI render test" style="rounded=1;whiteSpace=wrap;html=1;" vertex="1" parent="1"><mxGeometry x="20" y="20" width="160" height="60" as="geometry"/></mxCell></root></mxGraphModel>')
         echo "Render HTTP ${STATUS}, $(wc -c < /tmp/export-test.png) bytes"
+        # Puppeteer/Chrome render errors are logged by the worker process at
+        # request time, so the pre-render 'docker logs' above cannot capture
+        # them. Dump logs (and the response body) after the render so a 500
+        # surfaces the actual exception + stack in the CI output.
+        echo "=== export-server logs (post-render) ==="
+        docker logs image-export
+        echo "=== render response body (first 500 bytes) ==="
+        head -c 500 /tmp/export-test.png; echo
         test "${STATUS}" = "200"
         test "$(head -c 8 /tmp/export-test.png | od -An -tx1 | tr -d ' \n')" = "89504e470d0a1a0a"
         echo "${DOCKER_PASS}" | docker login -u "${DOCKER_USER}" --password-stdin;