The main difference between tagged version of the image is the installed PHP version.
The interpreter is run as usual with php. Pretty much all common modules are installed and a bunch of less common ones as well.
Latest Apache available in the Ubuntu repository is installed and set up to run by default if no command is given when running the container.
SSMTP is a simple sendmail implementation that sends your messages to the configured SMTP server. You can use an environment variable to configure this dynamically, or hardcode it in your image when you're building it.
You can set the environment variable SMTP in Docker. Entrypoint script will set that as the SMTP host for SSMTP.
PHP command mail() uses sendmail to send mail, so you if you're using it you will need to set this.
Any number of environment variables that you set in Docker can be passed to Apache and PHP. All you have to do is list the variable names in the APACHE_VARS environment variable. Use space as a delimiter.
The only enabled vhost file is /etc/apache2/sites-available/000-default.conf and you can easily override it when building your image if you need to. The default vhost points to the /app dir.
mod_remoteip is configured in /etc/apache2/conf-available/remoteip.conf. It is configured to trust the internal Docker network and to load the trusted-proxies.lst file.
Config file /etc/apache2/trusted-proxies.lst contains a list of IP ranges that are trusted for mod_remoteip. You should have your load balancers listed here if you want the Apache IPs to be correct. The trusted-proxies.lst file can be overwritten by setting the APACHE_TRUSTED_PROXIES environment variable.
If you create and make executable /usr/local/bin/apache_init.sh it will be run by the entrypoint script right before Apache is started. You can use this to run migrations or anything else your app needs to work.