I was thinking of planting a dwarf citrus tree in a new galvanized trash can. I was wondering if this is safe and if chemicals and or toxic substances can be drawn in systemically and transported to the fruit thereby potentially being consumed with the fruit?
Yes you can plant a citrus in a galvanized can, the zinc will eventually leach into the soil inside the can, it won't happen so fast that it would be problematic.
Zinc is one of the nutrients plants need anyway, what ever moves into the soil will be beneficial.
The second part of your post describes how roots work, what ever is in the soil that the roots can suck up, will be sucked up.
What goes into the roots goes up into the plant, so yes, what the roots take in will end up in the fruit, the leaves, the stems, the trunk.