fix(docker): harden default SSRF proxy egress

[laipz8200]

Summary

• Harden the Docker Compose SSRF proxy defaults by denying loopback, private, CGN, link-local, ULA, multicast, reserved, and cloud metadata target networks before general egress is allowed.
• Stop relying on the Ubuntu Squid image's packaged conf.d access rules so allow localnet cannot bypass Dify's policy.
• Preserve the sandbox reverse proxy path and add explicit SSRF_PROXY_ALLOW_PRIVATE_IPS / SSRF_PROXY_ALLOW_PRIVATE_DOMAINS escape hatches for trusted private-network deployments.

Screenshots

N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've updated the documentation accordingly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.94%. Comparing base (06ea0f7) to head (a7726f0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #36332   +/-   ##
=======================================
  Coverage   85.94%   85.94%           
=======================================
  Files        4454     4454           
  Lines      211374   211374           
  Branches    39507    39507           
=======================================
  Hits       181667   181667           
  Misses      26499    26499           
  Partials     3208     3208           

Flag	Coverage Δ
api	85.46% <ø> (ø)

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