How to use Apache httpd as a reverse proxy with Docker Compose and the "whoami" container?

I'm trying to set up a Docker Compose environment that includes an Apache httpd server and a "whoami" container, and I want to use the Apache httpd server as a reverse proxy to route requests to the "whoami" container.

Here's my docker-compose.yml file:

version: '3'

services:
  httpd:
    image: httpd:latest
    ports:
      - "80:80"
    volumes:
      - ./httpd.conf:/usr/local/apache2/conf/httpd.conf:ro

  whoami:
    image: containous/whoami
    ports:
      - "8080:80"

And here's my httpd.conf file:

LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule lbmethod_byrequests_module modules/mod_lbmethod_byrequests.so

LoadModule mpm_prefork_module modules/mod_mpm_prefork.so

ServerName 127.0.0.1
Listen *:80

<Proxy balancer://whoami>
    BalancerMember http://whoami-replica-1:80
    ProxySet lbmethod=byrequests
</Proxy>

ProxyPass / balancer://whoami
ProxyPassReverse / balancer://whoami

When I run docker-compose up, I can access the "whoami" container at http://localhost:8080, but when I try to access the Apache httpd server at http://localhost, I get httpd exited without any logs.

What am I doing wrong? How can I configure Apache httpd to act as a reverse proxy for the "whoami" container in Docker Compose?

httpd is exiting because your httpd.conf is invalid. You're replacing the default httpd.conf, which means you need to take care of mandatory configuration like loading an appropriate security module (e.g., mod_unixd).

A better approach is to copy the httpd.conf from the httpd image:

docker run httpd:latest cat /usr/local/apache2/conf/httpd.conf > httpd.conf

And then modify it to meet your needs.

I was only able to determine the problem by running Apache locally under control of strace:

$ sudo strace -e trace=write -s 1024 httpd -DFOREGROUND -f $PWD/httpd.conf
write(2, "[Tue Jun 06 06:00:33.356982 2023] [core:crit] [pid 755528:tid 755528] AH00136: Server MUST relinquish startup privileges before accepting connections.  Please ensure mod_unixd or other system security module is loaded.\n", 219) = 219
write(2, "AH00016: Configuration Failed\n", 30) = 30
+++ exited with 1 +++

Searching for that error message leads to this 2.2 to 2.4 upgrade documentation which among other things mentions the required mod_unixd module.

I was not able to come up with any command line invocation of httpd that would get it to emit this error in a useful fashion.