Try to fix the digest error.

[dj-maisy]

What?

We're getting a somewhat cryptic error message:

buildx failed with: ERROR: failed to solve: failed to push ghcr.io/alphagov/govuk-ruby-base:3.3.0: can't push tagged ref ghcr.io/alphagov/govuk-ruby-base:3.3.0 by digest

I'm not sure if this is because it is already trying to push the image with a tag before it has consolidated the different architecture images from across the different builds. But let's see if it's because push-by-digest doesn't replace/override the push option (these options appear to be poorly documented)