Security issue when updating Docker exposes setup page including SMTP username and password

Hello!

I've been using Cachet for awhile and love it. I found a potential security problem, though.

With the Cachet Docker container running, when updating Docker itself (not the image), for a brief few seconds, my Cachet URL `status.mydomain.com` redirects to the `/setup` page, and it **reveals my mail server's SMTP username and password in full view** (password visible using DevTools inspect)...


After about 10 seconds when the update of Docker is complete, going to the same page `status.mydomain.com` shows up normally.

This happened a few weeks ago but I thought maybe it was just a glitch. I just updated today and had the same thing happen.

As a workaround, I've blocked access to the `/setup` page in Cloudflare so that when this happens again it won't show the page.

Using Debian 10 Buster, not sure if it affects others.

**Steps to reproduce:**
1.) Go to the instance URL and continue to refresh over and over while performing the next steps
2.) Update Docker through `apt update;apt upgrade docker-buildx-plugin docker-ce docker-ce-cli docker-compose-plugin -y;`
3.) For a brief moment during the update, you should see the URL redirect to `/setup` and show the credentials

I'm not sure if it's all, some, or `docker-ce` that causes this, but those were the packages I updated.