Joseph is called the dreamer because of him having dreams which shows that he would be praised by others. He dreams of having himself as a person whom other people follow and praise as if by king.
~JZ Sy~
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When he was still a child, Joseph often dreamed dreams which could tell the future. Now, said future is set in an unknown timeline. This could be soon or in the years to come. These dreams aren't that obvious and so, Joseph can also say what are the meanings of these dreams. It is not only his own dreams that he could give meaning to, but also of others.
The cup bearer (or the wine taster for the pharaoh) and the baker in jail with him.