build/bake: skip registry identity validation without registry login

[crazy-max]

relates to https://github.com/docker/cli/actions/runs/30449872409/job/90569130577#step:3:173

Error: Invalid registry-identities input: registry-identities[0].connection_id must be a non-empty string

This change prevents setup-registry-identities from failing non-push image builds when registry login would not run under the current workflow logic.

The setup workflow now accepts a registry-login boolean and skips dependency installation plus registry identity validation when it is false. The build and bake workflows pass inputs.push && inputs.output == 'image', which matches the current condition used for registry login.

A caller can pass Docker Hub OIDC registry identities with connection_id: ${{ vars.FOO }} while running with output: image and push: false. If that variable resolves to an empty value, validation used to fail even though registry login was disabled and the identity would never be used. This keeps validation strict when login can happen, but avoids failing builds for unused registry identity config.

Also add extra commit to report errors cleanly with core.setFailed.