Comment # 6 on bug 1084818 from Ludwig Nussel

Note the third column in crypttab is not the password itself but rather the
path to a file. Upstream dracut has no extra code to include the keyfile in the
initrd, it assumes that the keyfile points to a device. So if you want your
password to be stored in a file in initrd you have to hook into dracut to
actually copy the keyfile to the initrd.