Strong's Greek Concordance defines δοῦλος (including a list of references from the NT).
"δοῦλος," Blue Letter Bible, accessed on October 4, 2023
a slave, bondman, man of servile condition
a slave
metaph., one who gives himself up to another's will those whose service is used by Christ in extending and advancing his cause among men
devoted to another to the disregard of one's own interests
a servant, attendant