Just to clear up this point, bonienl is referring to the way you defined TRANS_DIR as a variable and not as a path.
You make that selection as the bottom of the docker edit screen when you chose Add another Path, Port, Variable, Label or Device. You defined the mapping as a variable when it should have been a path.
For reference, below are some of my Plex host path mappings (note that /transcode is defined as a Container Path whereas in your screen shot TRANS_DIR is defined as a Variable: