mysqld: unknown variable 'defaults-file=/data/mysql/my.cnf'

It seems to be impossible to pass a configuration file to mysqld on startup: E.g.:

/usr/bin/mysqld_safe --basedir=/usr --datadir=/data/mysql/main --defaults-file=/data/mysql/my.cnf --pid-file=/tmp/my.pid --socket=/tmp/my.sock

results into:

/usr/sbin/mysqld --basedir=/usr --datadir=/data/mysql/main --plugin-dir=/usr/lib/mysql/plugin --defaults-file=/data/mysql/my.cnf --pid-file=/tmp/my.pid --socket=/tmp/my.sock --port=3306 --log-syslog=1 --log-syslog-facility=daemon --log-syslog-tag=

which in turn results into an error:
...
2017-05-09T22:16:32.748719Z 0 [Note] InnoDB: 5.7.18 started; log sequence number 2536517
2017-05-09T22:16:32.749035Z 0 [Note] Plugin 'FEDERATED' is disabled.
2017-05-09T22:16:32.753430Z 0 [ERROR] unknown variable 'defaults-file=/data/mysql/my.cnf'
2017-05-09T22:16:32.753597Z 0 [ERROR] Aborting