This needs to be added to apache-Dockerfile-block-1 at the top level, preferably right before we start messing with the .conf file, ie

It should also have a suitable comment something like # several Debian-supplied Apache configuration files assume the availability of certain environment variables (specifying generically in this way in case we run into more issues with this in the future).

An alternative worth considering (IMO) is to adjust apache2-foreground to source /etc/apache2/envvars similar to what the init.d script does, although we'll want to verify that the defaults in that file match what we've configured in our own apache2.conf file.

I agree with @tianon , source /etc/apache2/envvars should be added to apache2-foreground script. I faced the same situation, I added a vhost which need to specify the log directory to {APACHE_LOG_DIR}/xxxx-access.log, however, without the source statement, I got following error:

AH00111: Config variable ${APACHE_LOG_DIR} is not defined

I have to create a copy of apache2-foreground with the source modification to make it boot.