Add a Docker workflow that publishes multi-arch images

Pushes :edge for main and :x.y.z, :x.y and :latest for release tags,
optionally mirrored to another registry. The release workflow no longer
builds the image itself, so a tag does not publish it twice. The README
covers running the image anywhere and the settings the workflows read.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -1,6 +1,6 @@
 name: Release
 
-# Push a tag such as v0.2.0 to build and publish a release:
+# Push a tag such as v0.2.0 to publish a release. The Docker workflow builds the image from the same tag:
 #   git tag -a v0.2.0 -m "spinner v0.2.0" && git push origin v0.2.0
 on:
   push:
@@ -37,26 +37,3 @@ jobs:
         env:
           TOKEN: ${{ secrets.RELEASE_TOKEN || secrets.GITHUB_TOKEN }}
         run: sh deploy/publish-release.sh "${GITHUB_REF_NAME}"
-
-      # Skipped when the repository variable PUBLISH_DOCKER is "false", or fails alone
-      # if this runner has no Docker: the release above is already published by then.
-      # Optional secret REGISTRY_TOKEN (and variable REGISTRY_USER): an access token
-      # with package write permission, for servers where the job token can't push.
-      - name: Build and push the container image
-        if: ${{ vars.PUBLISH_DOCKER != 'false' }}
-        env:
-          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN || secrets.GITHUB_TOKEN }}
-          REGISTRY_USER: ${{ vars.REGISTRY_USER || github.actor }}
-        run: |
-          host=${GITHUB_SERVER_URL#*://}
-          owner=$(echo "$GITHUB_REPOSITORY_OWNER" | tr 'A-Z' 'a-z')
-          image="$host/$owner/spinner"
-          echo "$REGISTRY_TOKEN" | docker login "$host" -u "$REGISTRY_USER" --password-stdin
-          # a container-driver builder can build several architectures in one go
-          docker buildx create --name release --driver docker-container --use >/dev/null 2>&1 || docker buildx use release
-          tags="-t $image:$GITHUB_REF_NAME"
-          case "$GITHUB_REF_NAME" in
-            *-*) ;;                       # pre-releases (v1.0.0-rc1) don't move :latest
-            *) tags="$tags -t $image:latest" ;;
-          esac
-          docker buildx build --platform linux/amd64,linux/arm64 $tags --push .