Enable building containerized Kit CLI and init container

[amisevsk]

Description

Add a Dockerfile that just builds the Kit CLI and includes it in a container. While this container can be used to quickly run Kit commands, e.g.

docker run -it --rm kitimage:latest unpack mymodel -d /home/user/test

it is also used as a base image for other images:

• The KServe Dockerfile is largely unchanged and kept in case someone wants to use it
• An additional Kubernetes-style init container is added and makes it easier to run Kit in an init container to unpack a modelkit to some path in a volume. This container also supports verifying signatures using cosign.

Still TODO:

• Decide which repo to store these images in
• Add CI configurations to build versioned images on release and next images for each commit
• Figure out better base images for containers than just alpine.

Linked issues

Closes #369