There are a few primary mechanisms for customizing the configuration of a container.
Many images will react to environmental variables to alter their behavior. Images which offer this functionality should document it in the README for the image. For an example see the DevTools Build image. When environmental configuration is available it can be triggered by specifying the appropriate value in the environments section of your docker compose file.
www: image: phase2/apache-php:php56 environment: # Change some core container settings PHP_MAX_EXECUTION_TIME: 45 # These enable debug/profiling support PHP_XDEBUG: "true" PHP_XHPROF: "false"
If the container image you are using doesn't allow for change via environmental variables, your next option is to override the configuration file using volume mounting. In this setup, you replace the configuration file inside a container with a file from your host machine.
www: image: phase2/apache-php:php56 volumes: # substitute in a special mime magic file because our project handles files # of special types - ./env/www/etc/httpd/conf/magic:/etc/httpd/conf/magic
Volume Mounts for confd
Phase2 images often use confd to template configuration from environment variable and other sources. The configuration file you may be trying to override might be generated on container initiation via confd. If this is the case, you'll need to override the template file from which the configuration file is created. If you find that your copy of the configuration file on your host machine is updated when you start a container this is likely the cause.
For exmaple, the xdebug configuration provided in our phase2/apache-php:php70 container is proivided by confd, so this is how you would override that configuration
www: image: phase2/apache-php:php70 volumes: # substitute a different confd template file into the image so confd will use the override on container boot - ./env/www/etc/confd/templates/xdebug.ini.tmpl:/etc/confd/templates/xdebug.ini.tmpl