pack build <app img> --buildpack <buildpack source> allows developer to build app with custom buildpack

[ssisil]

GIVEN I have v3 buildpack(s) that exists on my local machine
WHEN I run pack build <app img> --buildpack <buildpack ID> --path <path to source code>
THEN my application will build using the custom buildpack
AND return the following image sha

Image: example.com/myorg/repo@434a7be...

Notes

• ID of an existing buildpack supported in default builder - ie. io.buildpacks.java, io.buildpacks.ruby, io.buildpacks.go, etc
• Can use -b as shorthand for --buildpack
• Can pass in multiple -b or --buildpack and the buildpacks will be run in that order
• Use builder specified with --builder. If not specified, use default builder for default stack
• No stack selection is possible yet
• Provide new order.toml with single group of specified buildpacks

[jchesterpivotal]

Later story: validate that buildpack(s) exist(s) in order.toml