mz480: multitarget builds - part 1

[mzihlmann]

Fixes #480

Description

First part of multi-target builds. This change allows building of multiple stages, but so far still only pushing a single stage. This allows for use-cases where after build we want to run tests. So far these use-cases had to rely on --skip-unused-stages=false, which we're going to deprecate.

FROM base AS final

FROM final AS test

In this case we could now run --target=final,test. Note that the order is important. The first stage in that list is the stage we're going to push, but it doesn't necessarily need to be the last stage we're going to build.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[mzihlmann]

let's also add plan and the new "golden tests" here or in a part - 0

[mzihlmann]

@Bixilon I think that would cover your use-case?

[Bixilon]

oh yes, precisely!

[mzihlmann]

depends on #486 for its integration tests