Rename as the Docker's entrypoint and move into assets

assets/docker-entrypoint.sh

@@ -25,7 +25,7 @@ if [ -z $SEAF_LIBRARY_UUID ]; then echo "The \$SEAF_LIBRARY_UUID was not defined
 # Update the user ID, if the $UID changed.
 if [ "$UID" != "1000" ]; then
     usermod -u $UID $UNAME
-    # What the $UID already exists ?
+    # What if the $UID already exists ?
 fi
 
 # Change the group, if the $GID changed.