I expect it’s unlikely that the side of your shipping container is a solid object anyway so the suggestion of using the Solid Tools for this would be moot.
Open the container object for editing so you can select the geometry of the side of the container. In the component I downloaded from the 3D Warehouse the side panel is a component. Draw the opening shape and extrude it through the surface of the container. Think of the extrusion as a cutter or a hole punch. Select all of the geometry, right click on it, choose Intersect Faces>With Selection.
This will divide the surface into inner and outer regions. By putting the cutter inside the side component and using Intersect Faces>With Selection you’ll prevent unwanted intersections.
Erase the unneeded parts and you should be left with the outline of your opening.
Select the surface inside the outline and press Delete.
