Removing ignored files also removes the Dockerfile

[jeff-knurek]

Actual behavior
After this change: 6ef616b
there is the potential to delete the Dockerfile that in the next step of building is no longer in the context

To Reproduce
in the .dockerignore, add something like:

*
!bin
!scripts/*.sh

this results in:

INFO[0000] Removing ignored files from build context: [* !bin !scripts/*.sh] 
Error: please provide a valid path to a Dockerfile within the build context with --dockerfile

[priyawadhwa]

Hey @jeff-knurek , this issue should be fixed with #424, please comment on this thread or open another issue if you run into any more bugs. Thanks!