Allow for file exclusions based on entries in an .imgpkgignore file - similar to .gitignore

[trisberg]

Describe the problem/challenge you have
[A description of the current challenge that you are experiencing.]

I'm creating an image for the contents of a git repo, would like to exclude some files/directories when the image is generated - I can try to use --file-exclusion but it would be convenient to have something like a .imgpkgignore file, similar to .gitignore or .tiltignore

Describe the solution you'd like
[A clear and concise description of what you want to happen. If applicable a visual representation of the UX (ex: new CLI argument name, the behavior expected).]

it would be convenient to have something like a .imgpkgignore file, similar to .gitignore or .tiltignore where I could specify these file exclusions

Anything else you would like to add:
[Additional information that will assist in solving the issue.]

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[joaopapereira]

Hey, @trisberg
This does make sense and definitely a cool feature.

In terms of syntax, I agree we should go with the common format that currently is used by gitignore.

Some implementation ideas:

• When reading the file we can generate the rules and use the same mechanism we have for --file-exclusion
• Having multiple .imgpkgignore can be allowed. Same behavior as git, the .imgpkgignore will be local to the folder provided
• We should fail if .imgpkgignore is ignoring the .imgpkg folder