Allow if-not-present pull policy for builds with image extensions

[MoritzWeber0]

Description

When running pack build and the builder contains image extensions, the build fails with:

ERROR: failed to build: pull policy must be 'always' when builder contains image extensions

Proposed solution

The pack build command should accept --pull-policy if-not-present as argument, also if the builder contains image extensions.

Additional context

I don't want to rely on a local registry or internet connection when building my images locally.

[jjbustamante]

Make sense to me, but no sure if @natalieparellano has some thoughts about it

[natalieparellano]

The issue with --pull-policy if-not-present is that we will not get a manifest for an image that only exists in the daemon (though with newer versions of docker using containerd as the backing store, that will hopefully one day be possible).

However, I believe some recent changes to imgutil (see https://github.com/buildpacks/imgutil/blob/main/local/v1_facade.go) should make it possible to construct a dummy manifest that may be good enough for our purposes (kaniko). We should do an exploration for it.

[hhiroshell]

Could you please assign this to me? This issue involves a slightly deeper implementation of the pack than those I've worked on before, but I’d like to take on more challenging tasks.

Anyway, I'm thinking about start with this exploration. I’d appreciate any advice you could share.