Add a docker ignore file so we don't copy over all binaries or extra files when building the image

[benjaminkostiuk]

Quality of life improvement that if people submit extra files with their submission we don't copy them over if they are generated or binaries or crap.

[OZoneGuy]

A great gitignore site here. You can look at different gitignore rules for different languages, IDEs, etc.

Maybe something like this can be used to ignore bloat files.