Use quotation when the path contain blank

[yufree]

When the path contains blank, the render_docker would fail. I suggested to add a test for blank in path. When the path contains blank, use extra quotation for the path of Dockerfile.

For example:

$docker_build_cmd
[1] "docker build --no-cache=false --rm=true  /Users/hehe/Box Sync/xcmsrocker/ISMMS"

This command would fail since the blank in the path would break the build and the right one should be

$docker_build_cmd
[1] "docker build --no-cache=false --rm=true  "/Users/hehe/Box Sync/xcmsrocker/ISMMS""

Also, I am curious about why you use \" \" for the tag and a default tag with lastest might be fine in most cases.

ps. I know blank in path is not cool and I have no idea why Box use such path...

[nanxstats]

@yufree -good catch! Let me try to find some time this week and fix it.