If the container is small relative to wavelength, then it won't have much effect at all.
If it's very large (say, infinite) then it will reduce ground losses by providing a more conductive ground plane. It doesn't matter if it's actually connected to the soil or not. This is a good thing.
The container will be closer to the antenna, which will change the spacing of the dipole and the image antenna under it, which may move the elevation of the main lobe up or down depending on the height of the dipole.
Since a storage container is probably neither very small nor very large relative to a 40m wavelength, you can expect effects somewhere between the extremes. In all, I'd say as long as the dipole isn't very close to the container you can expect no significant detrimental effects, and perhaps even some positive ones.
If you want to know more exactly what will happen, of course the thing to do is model the precise geometry of your situation.