push with a write:package token, not the Actions token

Second build failed the same way: docker login to this instance's registry
returns "unauthorized" for the automatic Actions token, and declaring
permissions: packages: write did not change it -- Gitea's task token is accepted
by the API but not by the package registry.

So the push uses REGISTRY_TOKEN, an org secret holding a personal access token
scoped to write:package and nothing else. The step fails with that sentence in
the log rather than another bare "unauthorized", because the difference
between a missing secret and a rejected credential is not visible from the
error docker prints.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CYFnHida4AdiNDek2pzgXx

.gitea/workflows/docker.yml

@@ -35,7 +35,17 @@ jobs:
         if: github.event_name != 'pull_request'
         run: |
           set -eu
-          echo "${{ secrets.GITEA_TOKEN }}" | docker login git.ffaerber.duckdns.org -u "${{ github.actor }}" --password-stdin
+          # Not the automatic Actions token: this Gitea's package registry rejects
+          # it (docker login returns a bare "unauthorized"), with or without a
+          # permissions: packages: write block. REGISTRY_TOKEN is an org secret
+          # holding a personal access token scoped to write:package only.
+          if [ -z "${{ secrets.REGISTRY_TOKEN }}" ]; then
+            echo "REGISTRY_TOKEN is not set — create a token with the write:package scope" >&2
+            echo "and add it as an org secret on token-for-token. The image built fine;" >&2
+            echo "only the push needs it." >&2
+            exit 1
+          fi
+          echo "${{ secrets.REGISTRY_TOKEN }}" | docker login git.ffaerber.duckdns.org -u "${{ vars.REGISTRY_USER || 'ffaerber' }}" --password-stdin
           docker tag "$IMAGE:$SHA" "$IMAGE:latest"
           docker push "$IMAGE:$SHA"
           docker push "$IMAGE:latest"