Complement test image: capture logs from nginx (#14063)

Have nginx send its logs to stderr/out, so that we can debug
#13334.

changelog.d/14063.misc

@@ -0,0 +1 @@
+Complement test image: capture logs from nginx.

docker/Dockerfile-workers

@@ -40,7 +40,11 @@ FROM matrixdotorg/synapse:$SYNAPSE_VERSION
     COPY --from=deps_base /etc/nginx /etc/nginx
     RUN rm /etc/nginx/sites-enabled/default
     RUN mkdir /var/log/nginx /var/lib/nginx
-    RUN chown www-data /var/log/nginx /var/lib/nginx
+    RUN chown www-data /var/lib/nginx
+
+    # have nginx log to stderr/out
+    RUN ln -sf /dev/stdout /var/log/nginx/access.log
+    RUN ln -sf /dev/stderr /var/log/nginx/error.log
 
     # Copy Synapse worker, nginx and supervisord configuration template files
     COPY ./docker/conf-workers/* /conf/