Adds Dockerfile to simplify compilation and testing.

[wernight]

Updated the README so it should be self-explanatory.

The Dockerfile builds the native binary for Linux and the Wasm, and it serves the web page to test the Wasm locally.

Note that Linux binary isn't fully working for me because it says:

0:1(1): error: syntax error, unexpected NEW_IDENTIFIER
                                                                                                                                  
compile shader failed : No OpenGL context has been made current

Seems that it needs some error checking to debug these issues a bit. See also https://stackoverflow.com/questions/21354301/glsl-syntax-problems-unexpected-new-identifier