Draft: Feature/manifest

First take on #1

add functionality to handle multiarch manifests

Adds an opinionated approach to create multiarch manifests.

It works by assuming the arch be attached to the separated with a dash. E.g. mytest-latest:1-arm64.

Adds a test for multiarch builds. The images are not really multiarch but tagged in a way that simulates the manifest creation behavior.

New flag --local is for local builds, mainly for testing purposes.

Multiarch manifests have to be pushed differently than normal tags, which has to be considered in CI/CD or in another script.

Caveat: The current test implementation does not work for several reasons:

• podman manifest add automatically treats image references as remote and tries to find them in a registry (which obviously does not work on localhost images). A different transport could be used, but podman only supports dir, archive and daemon as transports [1]
• if the manifest contains local references "podman images" fails with a non-zero exit code, hence the results can't be evaluated

[1] http://docs.podman.io/en/latest/markdown/podman-pull.1.html