feat: Unconditionally produce debug comments

Rationale for eliminating the check to see if the `DEBUG` environment
variable holds a true value:
  * The `DEBUG` environment variable might be set on a container (for
    purposes specific to that container, not `nginx-proxy`) to a value
    that cannot be parsed as a bool, which would break `nginx-proxy`.
  * It simplifies the template.
  * It eliminates a cold code path.
  * It avoids heisenbugs.
  * It makes debugging easier for users.

Also delete the debug info tests, as they are fragile and they provide
limited value.

Alternatively, we could avoid collision with the container's use of
the `DEBUG` environment variable by using a container label [1] such
as `com.google.nginx-proxy.nginx-proxy.debug`.  I think doing so has
dubious value, especially if we want to attempt backwards
compatibility with the `DEBUG` environment variable.

Fixes #2139

[1] https://docs.docker.com/engine/reference/commandline/run/#-set-metadata-on-container--l---label---label-file

Co-authored-by: Nicolas Duchon <nicolas.duchon@gmail.com>

README.md

@@ -490,12 +490,13 @@ Please note that using regular expressions in `VIRTUAL_HOST` will always result
 
 ### Troubleshooting
 
-In case you can't access your VIRTUAL_HOST, set `DEBUG=true` in the client container's environment and have a look at the generated nginx configuration file `/etc/nginx/conf.d/default.conf`:
+If you can't access your `VIRTUAL_HOST`, inspect the generated nginx configuration:
 
 ```console
-docker exec <nginx-proxy-instance> cat /etc/nginx/conf.d/default.conf
+docker exec <nginx-proxy-instance> nginx -T
 ```
-Especially at `upstream` definition blocks which should look like:
+
+Pay attention to the `upstream` definition blocks, which should look like this:
 
 ```Nginx
 # foo.example.com