FIXES #201 Sail browser extension docker not found error on MacOS

[dougnukem]

FIXES #201 chrome extension: failed to run docker info

On MacOS when Chrome is launched from Finder (as opposed to from terminal/shell session) the $PATH environment variable does not contain /usr/local/bin which is the common location for docker to be installed.

This PR adds in /usr/local/bin to be included in the PATH env variable on MacOS darwin.

See similar issues with other browser extensions that rely on Go to execute commands on the user's path:

browserpass/browserpass-legacy#13 browserpass/browserpass-legacy#159

[dougnukem]

/cc @deansheather

[dougnukem]

@deansheather accepted PR suggestions