Fix build from fork

[erikbosch]

We do not need builds from Pull Requests to be uploaded to ghcr. By removing builds from PRs we solve the problem that PRs originating from a fork cannot upload images to ghcr.

Fixes #70
Replaces #69

Fixes also some CI warnings and improves dash printout

Tested that pusihing to main still creates ghcr image at https://github.com/erikbosch/devcontainer-base-images/pkgs/container/devcontainer-base-images%2Fbase

[BjoernAtBosch]

What about if one wants to test a new base image before releasing it on main?

[erikbosch]

What about if one wants to test a new base image before releasing it on main?

Then there are two ways.

• You can push to "main" on the "from" repo, that will trigger a build and push. Off course this should not be used if you are a committer and uses the official velocitas repo as "from" repo
• You can trigger the action manually for your branch on the "from" repo.

I have tested both of these mechanisms, see https://github.com/erikbosch/devcontainer-base-images/pkgs/container/devcontainer-base-images%2Fbase