Temporarily force v2.8.1 (and some unrelated cleanup)

[PaulSD]

PrusaSlicer 2.9.0 dropped support for AppImage, and the "replacement" Flatpak builds are not compatible with Docker (and probably wouldn't support VirtualGL without lots of hacking even if you got past the Docker issues).

I suspect the best way forward would be to convert this to a two-stage docker build, where the first stage compiles the latest PrusaSlicer from source (in an Ubuntu container), then the second stage builds the runtime image with the compiled copy of PrusaSlicer. However, that isn't a trivial project.

For now, I simply hard-coded v2.8.1, so that the AppImage-based container can continue to be built for v2.8.1

I also cleaned up a few commands to eliminate some warnings when building.

[terhorst]

However, that isn't a trivial project.

@PaulSD I might have time to take this on. The compilation instructions appear pretty routine to me:

https://github.com/prusa3d/PrusaSlicer/blob/master/doc/How%20to%20build%20-%20Linux%20et%20al.md#step-by-step-guide

Just making sure I am not missing anything?