[builder] add imgpkg as builder

[cppforlife]

github.com/k14s/imgpkg is a builder that can create images from a set of files. let's have that as a backend. there probably needs to be two options: regular or bundle.

for bundles, i think we should have an option to run kbld to update images.yml

[voor]

This concept just blew my mind.

So instead of imgpkg push I would have a kbld configuration in my package cr that says "go to this folder for your bundle" and it would generate the bundle right there?

[cppforlife]

So instead of imgpkg push I would have a kbld configuration in my package cr that says "go to this folder for your bundle" and it would generate the bundle right there?

right, for convenience however we would probably want to run kbld on the bundle itself (aka recursive call), but that requires a bit of gymnastics in non-trivial cases (e.g. kbld <(helm template ...)) and then we start getting into business of running other commands (which isnt a business i think we should be in).

[voor]

With helm we probably need better tutorials and documentation on how vendir can help and kapp-controller will do the helm template step for you.

[cppforlife]

kapp-controller will do the helm template step for you.

yeah, but we still need kbld to collect images to write out .imgpkg/images.yml at the build time.

[voor]

kapp-controller will do the helm template step for you.

yeah, but we still need kbld to collect images to write out .imgpkg/images.yml at the build time.

:exploding_head: You're totally right, now I'm not sure how this will work