Upload crossbuild image to image repository

[AnHeuermann]

Issue

We could upload the crossbuild image to one or all of:

1. DockerHub: https://hub.docker.com/u/openmodelica

   • Create new repository openmodelica/crossbuild

2. GitHub Image registry: https://github.com/OpenModelica/openmodelica-crossbuild/pkgs/container/crossbuild

   • We already push images from PRs to there. But they are tagged differentyl.

   • You can already pull the image for the main version and all tagged versions:

     docker pull ghcr.io/openmodelica/crossbuild:main

3. OpenModelica container registry: docker.openmodelica.org

[AnHeuermann]

@adrpo What do you think? Adding it to DockerHub seems a good idea. In that case you would need to create a new repository in the openmodelica name space.

[AnHeuermann]

I signed and uploaded to the GitHub Container Registry. We can add other registries later on.