pack should have a pull policy of "try always"

[natalieparellano]

Description

Pull policy governs the pull behavior WRT ALL images used in the build - builder, buildpack packages, lifecycle image, run image

Today we have:

• always - try to pull, and fail if image is not found in the registry
• never - never pull
• if-not-present - only try to pull if the image doesn't exist locally

Proposed solution

We should also have:

• try-always (or something) - try to pull, fall back to local copy if present, and fail if image is not found in the daemon

This will cover cases where, e.g,. the builder is some local image that only exists in the daemon, but we want to make sure that we have the latest version of all other image inputs.

Describe alternatives you've considered

Do nothing

Additional context

• This feature should be documented somewhere

[natalieparellano]

Also discussed:

• best-effort
• try