docs: add proxy troubleshooting section to Docker deployment page

[brandonrc]

Context

Issue #395 on the backend repo reported persistent 503 health check failures for all internal services (meilisearch, trivy, openscap, dependency-track). Root cause was corporate proxy settings (HTTP_PROXY/HTTPS_PROXY) leaking into containers and routing internal traffic through the proxy.

This is a common scenario in enterprise and government environments. The Docker deployment page at docs/deployment/docker.mdx has an existing troubleshooting section but doesn't cover proxy-related issues.

What to add

Add a troubleshooting entry to the Docker deployment guide covering:

1. Symptom: All services show as unhealthy (503 Service Unavailable) in the dashboard and backend logs, even though individual containers are running and healthy
2. Cause: Host-level HTTP_PROXY/HTTPS_PROXY vars propagated into the backend container, causing internal health checks to route through the proxy
3. Diagnosis: How to identify this (look for external IPs in tcpdump on the compose bridge, check env | grep -i proxy inside the backend container)
4. Fix: The compose files now set no_proxy for internal service names (PR fix: add no_proxy to backend for container-to-container traffic artifact-keeper#413). For users on older versions, manually add service hostnames to NO_PROXY
5. Prevention: Note about .env proxy configuration and the .env.example documentation

Could also be worth adding a general "Corporate Proxy" section since Dependency-Track has its own proxy settings (ALPINE_HTTP_PROXY_*) and users behind proxies will need to configure both.

References

• Issues with backend communication artifact-keeper#395 (original issue)
• fix: add no_proxy to backend for container-to-container traffic artifact-keeper#413 (compose fix PR)