RProxy | RProxy is a reverse proxy server written with performance | Proxy library

The problem was exactly as @zigarn had pointed out in the comments. The apps being reverse proxied were relying on URLs with a base of / and not something like /statping or /homeassistant. Portainer worked because it apparently does not have that same dependency.

The solution is to move away from reverse proxying based on URL and instead use reverse proxy based on hostname. So now, rather than http://otto.local/statping/, I have http://statping.otto.local/ All reverse proxied apps load their home pages with this arrangement, so I feel like it's a win.

There area still some minor troubles with homeassistant and node-red, and I suspect this may be due to a need to reverse proxy web sockets. Still researching that, but the main objective of getting them all to pull up a home page or login page has been accomplished.

Now for the gory details...

Because this is a home setup, with the standard, basic functionality internet router, I had to put some work into setting up a DNS server before names like statping.otto.local or homeassistant.otto.local would resolve.

To do this, I installed bind 9 on otto, the same host running the Docker containers. It's installed from a package on Raspberry Pi OS rather than as a container. The trick was to use a wildcard CNAME in addition to the DNS entry for otto.local.

With *.otto.local pointing to otto.local, I am able to get any combination of {appname}.otto.local to resolve to otto's IP address. Now, typing http://portainer.otto.local et al. gets me to otto and Nginx.

In the Nginx config, it's a matter of creating several entries that look like this one: