Docker: rename image/container name from "webgoat" to "webgoat.net"

[pkrukp]

Closes #13

[tobyash86]

@pkruk2 have you checked that the new name including dot does not have any side effects?

[pkrukp]

I have checked that I can run it (using linux containers). Also, other images have dots and special characters, for example "mcr.microsoft.com/dotnet/core/sdk:3.1". Further looking at docker/compose#1519 the dot "." is allowed. I haven't found explicit documentation in docker about it.

[tobyash86]

Great, thanks!