external file domain blocked by agent squid proxy

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### Dify version

1.16.1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

Currently the squid conf allows internal api domain. If user specifies external file domain, access is blocked, i.e., skill upload will fail.

### ✔️ Expected Behavior

The configured file domains are included in squid conf and agent file upload should always succeed.

### ❌ Actual Behavior

_No response_

Start by locating the self-hosted Docker Squid configuration and the configured file-domain handling described in the report. Reproduce a skill upload with an external file domain, then verify that configured external domains are included in Squid and the upload succeeds.

Written by the indexing model from the issue text.