When pre-validating a custom nginx.conf.sigil before the build phase, no app listeners exist yet on first deploys. Templates that emit `proxy_pass http://app-port` while gating the matching upstream block on `DOKKU_APP_WEB_LISTENERS` render an undefined upstream, causing `nginx -t` to fail with "host not found in upstream". Pre-validation now passes a `127.0.0.1:5000` placeholder for `DOKKU_APP_WEB_LISTENERS` so the upstream block emits a static server entry and the template can be validated for syntax without depending on live listeners.
Renders the user-supplied nginx.conf.sigil via sigil into a tmp file and runs `nginx -t` against a wrapped copy as soon as the template is extracted from the source tree, so syntactically invalid templates abort the deploy before the build phase runs. Skipped when `proxy-type` is not `nginx`, when `disable-custom-config=true`, or when no custom template was extracted. Closes#7827.
Some files - those maintained by external organizations - have a very light treatment and solely point to the upstream documentation to reduce any issues creating examples/documentation for them that may differ in the future.
Closes#7315
