multistage-build: add support for --target option

[flixpae]

Is your feature request related to a problem? Please describe.
I try to build a custom ci-builder image to build container images in gitlab-ci on kubernetes

In docker or kaniko a --target option is present.
i would like to control the build target in makisu in a similar way

Describe the solution you'd like
a behaviour close to what the docker documentation describes
https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds

Describe alternatives you've considered
Build with kaniko could work but multistage-builds are tricky and kaniko is not intended to be integrated in a custom image

[flixpae]

Where would i need to look in the code ?
maybe i can look into this a bit further ?

[yiranwang52]

@flixpae you will need to look into lib/builder/build_plan.go

Can you give us an example? I want to understand why using the last stage as target wouldn't work.