Ignore file check

[ain]

As by the story from #99, -i option could use validation. If the ignore file does not exist at the path, there should be an error (or prompt).

[cjerdonek]

FYI, erroring out or prompting at a missing file would break @brikis98's suggestion here to ignore a file that is already present. There is no other way that I know of to ignore a file.

So I would suggest that if erroring out, etc. is added as an enhancement, then an option to ignore a file that's already present be added at the same time.

[brikis98]

I'm not sure it makes sense to error out of a file you asked to ignore doesn't exist. Most other systems don't do this: e.g. .gitignore or .dockerignore. At most, a warning could be useful.

[cjerdonek]

I'm not sure it makes sense to error out of a file you asked to ignore doesn't exist.

To clarify, he's suggesting that it error out if the file you provide that specifies which files to ignore doesn't exist (e.g. the .dockerignore file) -- not if some of the files the .dockerignore file references don't exist.

[brikis98]

Ah, I misunderstood. That makes more sense.