Fix flaky image build

[psftw]

Note: the original GitHub repository was deleted recently, so we've lost all the issues/discussion 😢

Building this image is difficult due to issues with GnuPG and key servers. More details in https://github.com/docker-library/faq#openpgp--gnupg-keys-and-verification

The most straight-forward solution would be to directly embed the keys in this repository, though this is not ideal.

[phadej]

As far as I see, the gpg checks are used only for stack, would a solution to embed the sha512 sum into Dockerfiles instead?

It's a bit more work to update stack versions, but maybe better than flaky builds?

[psftw]

We have signed artifacts and keys so we might as well use them as intended vs checksums. It turns out, embedding the keys adds more complexity than I'm OK with ( psftw@465e064 ). Splitting up the single RUN command similarly without embedding keys would also help with flakiness, which seems like the right approach.

[psftw]

pushed up changes which split up the build to make fail faster which is the best we can do for now