Add image build workflow on prs and pushes

[fdegir]

This commit introduces a new workflow to attempt building EIB container image for prs and pushes to verify the image is buildable.

[jdob]

Dude, CI additions always make me happy :)

[e-minguez]

Just in case I had this in my own fork https://github.com/e-minguez/edge-image-builder/blob/build-container/.github/workflows/build-container.yml
This includes pushing the container image to the ghcr registry as well

In fact #237 but IIRC I closed it because there was no interest on having this at that time.

[fdegir]

Just in case I had this in my own fork e-minguez#1 This includes pushing the container image to the ghcr registry as well

Didn't know you were looking into this as well @e-minguez.

Not having an image build check for prs/pushes made me add this so I added a simple one.
We could and should perhaps have a push/post-merge workflow based on yours to push built images and update this one later on.