fix: omit Content-Length on 307 redirects when serving direct manifest for containers (#8037)

Containers have been refactored to use the same serve method as other packages, ensuring consistent response handling.

fixes #7888

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

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### Release notes

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<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Bug fixes
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8037): <!--number 8037 --><!--line 0 --><!--description b21pdCBDb250ZW50LUxlbmd0aCBvbiAzMDcgcmVkaXJlY3RzIHdoZW4gc2VydmluZyBkaXJlY3QgbWFuaWZlc3QgZm9yIGNvbnRhaW5lcnM=-->omit Content-Length on 307 redirects when serving direct manifest for containers<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8037
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Julian Schlarb <julian.schlarb@denktmit.de>
Co-committed-by: Julian Schlarb <julian.schlarb@denktmit.de>

modules/httplib/serve.go

@@ -35,6 +35,8 @@ type ServeHeaderOptions struct {
 	Filename           string
 	CacheDuration      time.Duration // defaults to 5 minutes
 	LastModified       time.Time
+	AdditionalHeaders  http.Header
+	RedirectStatusCode int
 }
 
 // ServeSetHeaders sets necessary content serve headers
@@ -82,6 +84,12 @@ func ServeSetHeaders(w http.ResponseWriter, opts *ServeHeaderOptions) {
 		// http.TimeFormat required a UTC time, refer to https://pkg.go.dev/net/http#TimeFormat
 		header.Set("Last-Modified", opts.LastModified.UTC().Format(http.TimeFormat))
 	}
+
+	if opts.AdditionalHeaders != nil {
+		for k, v := range opts.AdditionalHeaders {
+			header[k] = v
+		}
+	}
 }
 
 // ServeData download file from io.Reader