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.
63b809f64c