How to debug deployment to DXC of a hello world "alloy" app.

I setup VS 2019 as per EPiServer instructions and got Alloy working locally.

Our organisation setup a DXC subscription, and I was sent the publish details for the integration env.

I followed the exact instructions here:  https://world.episerver.com/digital-experience-cloud-service/deploying/creating-a-new-cms-site-and-deploying/

If I hit plublish, I see this in the output window:

2>Adding child sitemanifest (E:\dev\xxx\epi_projects\Alloy1\obj\Release\Package\PackageTmp).
2>Adding child sitemanifest (E:\dev\xxx\epi_projects\Alloy1\obj\Release\Package\PackageTmp).
2>Adding child sitemanifest (E:\dev\xxx\epi_projects\Alloy1\obj\Release\Package\PackageTmp\App_Data).
2>Publish Succeeded.
2>Web App was published successfully http://rnkg0xxxxxxxinte.dxcloud.episerver.net/

The publish was intant (less than 1 sec) which seems odd, however, the integration DXC site has gone from a dummy welcome page to the following:

Server Error in '/' Application.
Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.
Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".<!-- Web.Config Configuration File --><configuration><system.web><customErrors mode="Off"/></system.web></configuration>
Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.<!-- Web.Config Configuration File --><configuration><system.web><customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/></system.web></configuration>

I tried adding <customErrors mode="Off"/> to the existing system.web section in the existing Web.config, saved it, then rebuilt and replublished, but get same error.

I tried looking in application insights, but strangely, under deployments there is no record of my deployment, and under logs is nothing - no logs.

Under "activity log" and "quick insights" there is nothing for "failed deployemnts" nor "errors"

How can I debug this?  The publish from VS did someting (as now I get an error message on the integration website), but there seems to be no record of it, and no errors.

Any help appreciated.