How to shave off a fixed number of pixels from a PNG with a transparent background using ImageMagick

I want to use ImageMagick to go along the outside of any shape and shave off a desired number of pixels. The shave option only applies in the x and y directions. Whereas I need it to applying along the entire edge of the shape. Does anyone know if this can be done with ImageMagick? Would really appreciate some pointers. Even if it’s to a specific option. I can’t seem to find anything that will do it, but it’s such a powerful library that I would think it should be possible.

Thanks.