Increase CMake Version

[Squareys]

Hi @asria!

I am using your docker image to compile a project based on magnum which supports emscripten builds through a cmake toolchain.

The toolchain requires at least CMake 3.7, though (https://github.com/mosra/toolchains/blob/master/generic/Emscripten-wasm.cmake#L48 for explanation), but the image only provides 3.6.

I already built a custom version of your image where I simply updated the cmake download link and that works great, but it would be amazing to not have to maintain that myself with every emscripten update.

Would you be okay with increasing the CMake version? Which version would you suggest (3.7/3.10)? I can provide a PR, if you're fine with it.

Thanks in advance and thanks for actively maintaining this docker image!
Cheers, Jonathan.

[trzecieu]

Hi @Squareys,
I think that 3.7 will be a good choice. I'd be very grateful for a contribution, then it will be included to the new release.

Cheers,
Piotr