[cache-flag] Support build image cache

[jromero]

pack build --cache 'type=build;format=image;name=<image name/image registry address>'

Build cache as an image is currently only supports using cache as an image from registry which requires the final app image to be pushed to registry (The --cache-image and --publish flags need to be used together).

To add complete support for build cache as an image,
Support for scenarios will be implement -

1. Cache image used from registry when exporting app image to docker daemon.
   • Decouple --cache-image and --publish flags from pack, Store image in local docker daemon when using pack build --publish #1024
   • Add support for this kind of export to the lifecycle's export phase

Support for scenarios already exists, might require a bit of modification -

1. Cache image used from registry when exporting app image to a registry.
   • If an image is used as build cache, then it is fetched from and exported to a registry by default

Support for scenarios does not seem feasible from previous discussions, need confirmation for next steps on these -

1. Cache image used from local docker daemon while exporting app image to registry
2. Cache image used from local docker daemon while exporting app image to docker daemon
   • These do not make sense because build cache images do not belong to docker daemon and performance might take a hit in doing so, so it is better to stick to volume cache for local cache.

• Implement the flag support based on acceptance criteria defined below

Acceptance Criteria

Image is pushed to the registry

Given the current working directory contains source code
And configured builder can build source code
When user executes 'pack build myorg/myapp:latest --cache "type=build;format=image;name=myorg/myimage:cache" --publish'
And build finishes successfully
Then 'myorg/myapp:cache' should existing in registry
And 'myorg/myapp:cache' image should contain the cached build layers

Image build cache should require --publish flag

Given the current working directory contains source code
And configured builder can build source code
When user executes 'pack build myorg/myapp:latest --cache "type=build;format=image;name=myorg/myimage:cache"'
Then the build should fail
And an error containing the following should be present:
"""
invalid cache definition: 'image' format is only acceptable with '--publish' flag.
"""

[jromero]

Add support for this kind of export to the lifecycle's export phase

Could you elaborate on this? AFAIK, we'd be leveraging the existing lifecycle functionality.

[jromero]

Decouple --cache-image and --publish flags from pack

I think this is a little out of scope if we consider trying to solve #1024.

Could you explain the reasoning for this?