Simplify CMD: Remove shell parameter expansion, rely on ENV defaults

- Remove complex shell parameter expansion from CMD
- Use ENV instructions for defaults, envsubst for substitution
- Fixes GitHub Actions CI/CD pipeline failures
- Container now starts successfully with proper nginx config

Author: Robbie1977

Dockerfile

@@ -16,4 +16,4 @@ RUN mkdir -p /var/cache/nginx && chown -R nginx:nginx /var/cache/nginx && \
 
 EXPOSE 80
 
-CMD ["/bin/sh", "-c", "UPSTREAM_SERVER=${UPSTREAM_SERVER:-owl.virtualflybrain.org:80} CACHE_MAX_SIZE=${CACHE_MAX_SIZE:-20g} DNS_RESOLVER=${DNS_RESOLVER:-8.8.8.8 1.1.1.1} envsubst '${UPSTREAM_SERVER} ${CACHE_MAX_SIZE} ${DNS_RESOLVER}' < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf && /usr/local/bin/health-monitor.sh & nginx -g 'daemon off;'"]
+CMD ["/bin/sh", "-c", "envsubst '${UPSTREAM_SERVER} ${CACHE_MAX_SIZE} ${DNS_RESOLVER}' < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf && /usr/local/bin/health-monitor.sh & nginx -g 'daemon off;'"]