Rename images and push to ghcr.io

[matifali]

Rename image tags and push them to ghcr.io.

• images/base/ubuntu.Dockerfile -> ubuntu:base
• images/minimal/ubuntu.Dockerfile -> ubuntu:minimal
• images/$IMAGE/$DISTRO.Dockerfile -> $DISTRO:$IMAGE

Originally posted by @johnstcn in #275

[bpmct]

When we rename, let's just push twice for backwards-compatibility.

[matifali]

We plan to publish new images on ghcr.io only while keep pushing the old ones as is to dockerhub. Later we will add a MOTD to dockerhub ones and then stop updating them eventually.