An very easy fix came up to me for that problem:
Most of the shower tubs are made of steel covered by porcelain enamel, but still it is steel!
So what we need here are some nice magnets and some tape for the first version.
I used electrical tape because that stick quite good to my shower curtain made of vinyl.
The magnet is a neodym magnet - small and very strong!
I attached like four magnets to the curtain - one every 20 cm.
A possible improvement would be to somehow stitch the magnet to the curtain because the electrical tape does not like warm showers as I do. So it happened that some of the magnets detached again. But so far I couldn't find magnets with a hole in this size.
Hope you like this!-)