Builder panics if Dockerfile missing

[phemmer]

docker.go line 143 & 144 isn't checking err (Bad @bparees :-P)
Causes a panic if the repo has no Dockerfile in it.

[phemmer]

I can easily create a quick fix PR if desired. But if a test is needed, it won't be til tomorrow.

[bparees]

#860

[bparees]

Thanks for the keen eye.

[ironcladlou]

This should be closed by #860.