Fix parsing of docker options.

[Alexhuszagh]

Fix parsing DOCKER_OPTS to correctly parse them as shell words, rather than splitting on whitespace.

Closes #769.

[Alexhuszagh]

I think parsing them as a Unix shell is the best idea, since users are likely going to be using a Unix shell, it handles simple cases well, and even on Windows CMD this works identically for most default cases (unsure about Powershell), and you can almost always provide the correct arguments in the shell regardless.

[Emilgardis]

works in powershell too, I'm sure there's edge cases but this is better than being completely wrong most of the time

bors r+

[bors]

Build succeeded:

• conclusion