I don't know of any easy way to generate a variable thickness wall. I think you just have to generate the outer and inner walls separately. I would build the outer wall first, as that is the one that is most critical for appearance. Then use the same methods to build the inner wall, showing the sketches for the outer wall to help you generate the matching inner wall sketches.
Hey, but I don't design bottles, so maybe someone who really knows what they are talking about will show you the easy way.
I agree with Jerry, that you will need to create each wall independently.
I have done this before, and I do it by creating offsets at each distance, then using those edges for a boundary surface. I have done this before on aluminum castings with good results.
I have attached an example file for reference (SolidWorks 2013).
TransitionThickness_cc.SLDPRT.zip 277.6 KB
Jerry / Charles
Thanks for your input guys sounds like a way forward.
Unfortunately i could not open your file Charles as I am working with SW 2012... but thanks.