Update puppeteer healthcheck Dockerfile to run as root for Docker socket access and enhance README with common error resolution steps.

Dockers/puppeteer-healthcheck/Dockerfile

@@ -29,7 +29,8 @@ RUN groupadd -g 999 docker || true && \
     useradd -r -g docker -u 999 healthcheck && \
     chown -R healthcheck:docker /app
 
-USER healthcheck
+# Don't switch user - run as root for Docker socket access
+# USER healthcheck
 
 # Run the healthcheck script
 CMD ["python", "healthcheck.py"]