E-Mail attachment directory is by default in /kopano/data/attachments/ so bind /kopano/data as volume.
You can reconfigure by setting environment variable KCCONF_SERVER_ATTACHMENT_PATH.
You can change all server.cfg settings you like prefixed with KCCONF_SERVER_
So specify KCCONF_SERVER_MYSQL_HOST for mysql_host setting in server.cfg.
Or specify KCCONF_LDAP_LDAP_SEARCH_BASE to set ldap_search_base in ldap.cfg.
You may override default settings with KCCONF_* options or comment specific options in/out with KCCOMMENT_filenameWithoutExtension_anystring=searchline
e.g. KCCOMMENT_LDAP_1=!include /usr/share/kopano/ldap.openldap.cfg
For coredumps on crashes kopano-server requires the fs.suid_dumpable sysctl to contain the value 2, not 0.
The docker image kopano_ssl will create certificates for all containers. Those certificates are selfsigned and only for internal Kopano component communication.
kopano_webapp port 80 is meant to be published through a https reverse proxy. MAPI connection for Outlook is also handled over port 80.
Maybe you need to execute kopano-cli --list-users once after initial install in the kopano_server container.
Example:
docker-compose exec kserver kopano-cli --list-users (This may last very long without any console output.)