ci: simplify Dockerfile and add docker image testing

[galargh]

Dockerfile

This change splits the builder stage into builder and utilities stages. This is to make the distinction between buildtime and runtime dependencies clearer. More importantly, this makes sure that the runtime dependencies we download are for the target platform.

It also install tini and su-exec* from APT repository rather than GitHub. This makes the installs simpler because we don't have to checkout git repos anymore.

Finally, it replaces su-exec with gosu. This is because su-exec is not distributed through APT. However, gosu comes in at around 2MB while su-exec is ~20KB. Let me know if we should go back to su-exec for the size benefits.

GitHub Actions

This PR adds bare minimum testing of produced images. It adds it to both docker-build and docker-image workflows (Should we get rid of docker-build one?). The testing consists of running --version in the produced image.

In case of docker-image, I had to split docker-build-and-publish into build and publish phases. Furthermore, I had to split build into 3 separate builds for each platform. This is because we want all the produced images loaded and it's impossible to do with multi-platform build.

The testing in docker-image all happens on amd64 instance. So other platforms are being emulated.

Testing

• https://github.com/ipfs/kubo/actions/runs/5596879840

Fixes ARM images and:
closes #9901
closes #7541

[lidel]

Thank you for cleaning this up. Let's keep existing actions for now, adding this --version check is enough.
Sgtm, let's merge this, so we see how it behaves in real world (if it increases build time much) before final 0.22 is released.

[lidel]

@galargh just tested and confirmed kubo:master-2023-07-27-c5868a8 works fine on ARM 👍 thanks again for fixing this! 🙏