disallow dockerfile outside base dir #19
Description
Right now it's possible to use something like --base foo --dockerfile ../Dockerfile, but this breaks the promise of tree hash tagging. Once config file support is implemented (#14) this should be dissallowed.
Sometimes it's useful to be able to re-use same dockerfile with different context dirs, but this should be catered for with build configs, where would would be able to specify a simple dockerfile in-line, since the config file changes will re-trigger the build, it should provide the right semantics.