Warn about implicit contextDir change

[ctrlok]

© @ybogdanov

Why is the default behavior taking base dir of Rockerfile instead of a current working directory? Because this behavior is more intuitive than the opposite. You can run a Rockerfile from any directory while keeping its functionality without a need to specify the context directory repetitively. Docker solves the latter by making the last argument mandatory.
But yes, it's a good idea of making a warning if a base directory of Rockerfile is not same as current and the last argument is not specified explicitly.

[hitthefrei]

@ctrlok
direcotry -> directory

[ybogdanov]

I would say something like this:

log.Warningf("Implicit context directory used: %s. You can override context directory using the last argument.", contextDir)

[ctrlok]

@ybogdanov @hitthefrei Done.