Using $HOSTNAME in apache log file name - docker

Question

I need to run several docker containers running apache.
To centralize the log files in single location, I want to use the hostname of the docker container where apache is running (not the virtual server name) as a part of the log file name, i.e. I need something like /var/log/apache2/${APP_NAME}.access.${HOSTNAME}.log
I prepared a virtual host config like this:
<VirtualHost *:80>
...
...
...
ErrorLog /var/log/apache2/${APP_NAME}.error.log
LogLevel warn
LogFormat "%V - %{CLIENTIP}e %l %u [%{%d/%b/%Y %T}t.%{msec_frac}t %{%z}t] \"%r\" %>s %b %D \"%{Referer}i\" \"%{User-agent}i\"" extended
CustomLog /var/log/apache2/${APP_NAME}.access.${HOSTNAME}.log extended
</VirtualHost>
where $HOSTNAME and $APP_NAME are environment variables passed by the shell where apache is running.
This configuration seam to work, in the sense that when apache is running in docker container named ad331fa1 it creates a file named like /var/log/apache2/myappname.access.ad331fa1.log
But.... the timetaken to handle the request increased by 4-5 seconds per request!!
This happens only for requests that are handled by the php engine, while no delay is added when serving static files (.img, .css, etc...)
The problem disappears when using a config like this:
<VirtualHost *:80>
...
...
...
ErrorLog /var/log/apache2/${APP_NAME}.error.${HOSTNAME}.log
LogLevel warn
LogFormat "%V - %{CLIENTIP}e %l %u [%{%d/%b/%Y %T}t.%{msec_frac}t %{%z}t] \"%r\" %>s %b %D \"%{Referer}i\" \"%{User-agent}i\"" extended
CustomLog /var/log/apache2/${APP_NAME}.access.log extended
</VirtualHost
i.e. removing the $HOSTNAME solve the issue.
Note that both $APP_NAME and $HOSTNAME are environment variables, but the issue happens only when $HOSTNAME is used in the log file name.
Any suggestion?