The nginx deploy-time pre-validation runs `nginx -t` against a minimal wrapper that omits the global `load_module` directives, so a custom `nginx.conf.sigil` using a directive from a dynamically loaded module fails validation even though it is valid against the running server. Document overriding the `validate-config` template through the `nginx-app-template-source` trigger as the supported workaround.
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.
