Add run-image option for image building

[scottfrederick]

The pack CLI build command supports a run-image option that allows the user to override the run image specified in the builder stack metadata. This option could be useful to users of the Spring Boot Maven plugin spring-boot:build-image goal and Gradle plugin bootBuildImage task.

[slyoldfox]

@scottfrederick been experimenting for a day or so with buildpacks. Adding the run-image seems like a good enhancement.

At the moment I seem to be stuck having to create a custom builder, referencing the run-image even though I am not modifying anything else.

Also, is it correct that the builder and run-image need to exist in a remote registry? I was testing locally (without pushing) and was receiving a 404.

pack create-builder docker.io/foo/my-builder --builder-config ./builder/builder.toml

[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.3.0.RELEASE:build-image (default-cli) on project demo: Execution default-cli of goal org.springframework.boot:spring-boot-maven-plugin:2.3.0.RELEASE:build-image failed: Docker API call to 'localhost/v1.24/images/create?fromImage=docker.io%2Ffoo%2Fmy-builder%3Alatest' failed with status code 404 "Not Found" -> [Help 1]

Is this normaly behaviour or have I missed some documentation?

[Woodham]

This would also solve a problem for us, as we're required to pull images through a corporate docker mirror and there's no way of changing the run image name to be able to do that currently.