The docker image contains a set of Apache configuration files whose entry point is /etc/apache2/apache2.conf. Traditionally that file ends like this:

IncludeOptional conf-enabled/*.conf
IncludeOptional sites-enabled/*.conf

In the case of docker-mapserver, there is an extra ErrorLogFormat line at the end of that file. This means that ErrorLogFormat directives set in conf-enabled/ are ignored.

Could you please move that ErrorLogFormat somewhere that can be conveniently overridden? Either earlier in the apache2.conf file or in a separate file within conf-enabled/?