[SUREFIRE-1840] Distinguish commands in docs for testing with docker

[mslwang]

sudo, which provides root/ administrator level priviledges on *nix
systems, should be only used when required. Given that docker uses the
kernel of the host system for Linux while docker uses a virtual machine
for Windows/macOS, it is omitted for the latter. This was clarified for
the documentation concerning testing Maven Surefire with docker.

[Tibor17]

Thx for contributing.

[mslwang]

Update: A caveat I've discovered for this with some more exposure to docker is the usage of docker groups to avoid using sudo in Linux. It's a common practice, but not necessarily the safest, so I'm not sure what the best practice would be here.