They put him in custody, because there was no clear instruction about what should be done to him.

The captain of the guard appointed Joseph to be their attendant, and he served them. ¹  They spent some time in custody. ² 

So he asked Pharaoh’s officials, who were with him in custody in his master’s house, “Why do you look so sad today?” ¹ 

So they placed him in custody until they were able ¹  to make a clear legal decision for themselves based on words from the mouth of the Lord. ²