ci: derive registry host, owner and user for Docker publish

Login failed with "Username required" because REGISTRY_OWNER was not set.
Default the registry host to this Gitea instance, the owner to the
repository owner and the user to the tag pusher, lowercase the image
name, and keep a failed GHA cache export from failing the build.

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

.github/workflows/docker-publish.yml

@@ -5,11 +5,11 @@ name: Docker publish
 # Configure the repository by adding these secrets / variables:
 #   Secrets:
 #     REGISTRY_TOKEN  - Gitea PAT with `write:package` (and `read:repository`) scope.
-#   Variables (Settings -> Actions):
-#     REGISTRY_HOST    - the Gitea registry host, e.g. gitea.com (default below)
-#     REGISTRY_OWNER   - owner/namespace of the repo, e.g. your-username or your-org
-#     REGISTRY_REPO    - repository name, e.g. balance-server
-#     REGISTRY_USER    - the username owning the token if it differs from REGISTRY_OWNER.
+#   Optional variables (Settings -> Actions); defaults come from the repository:
+#     REGISTRY_HOST    - the Gitea registry host (default: host of this Gitea instance)
+#     REGISTRY_OWNER   - owner/namespace of the image (default: repository owner)
+#     REGISTRY_REPO    - image name (default: repository name)
+#     REGISTRY_USER    - the username owning the token (default: user who pushed the tag)
 #
 # NOTE: variable/secret names must NOT start with GITEA_ or GITHUB_.
 
@@ -20,10 +20,10 @@ on:
       - "v*"
 
 env:
-  REGISTRY_HOST: ${{ vars.REGISTRY_HOST || 'gitea.com' }}
-  REGISTRY_OWNER: ${{ vars.REGISTRY_OWNER }}
+  REGISTRY_HOST: ${{ vars.REGISTRY_HOST }}
+  REGISTRY_OWNER: ${{ vars.REGISTRY_OWNER || github.repository_owner }}
   REGISTRY_REPO: ${{ vars.REGISTRY_REPO || github.event.repository.name }}
-  REGISTRY_USER: ${{ vars.REGISTRY_USER || vars.REGISTRY_OWNER }}
+  REGISTRY_USER: ${{ vars.REGISTRY_USER || github.actor }}
 
 jobs:
   publish:
@@ -40,10 +40,19 @@ jobs:
         id: meta
         env:
           VERSION: ${{ github.ref_name }}
+          SERVER_URL: ${{ github.server_url }}
         run: |
           # Keep only semver body so "v1.2.3" -> "1.2.3".
           tag="${VERSION#v}"
           echo "version_tagname=$tag" >> "$GITHUB_OUTPUT"
+          # The Gitea registry lives on the same host as the instance itself.
+          host="${REGISTRY_HOST:-${SERVER_URL#*://}}"
+          host="${host%%/*}"
+          echo "host=$host" >> "$GITHUB_OUTPUT"
+          # Image references must be lowercase ("BalanceServer" -> "balanceserver").
+          image="$host/${REGISTRY_OWNER,,}/${REGISTRY_REPO,,}"
+          echo "image=$image" >> "$GITHUB_OUTPUT"
+          echo "Publishing $image:$tag as $REGISTRY_USER"
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -51,7 +60,7 @@ jobs:
       - name: Log in to Gitea Container Registry
         uses: docker/login-action@v3
         with:
-          registry: ${{ env.REGISTRY_HOST }}
+          registry: ${{ steps.meta.outputs.host }}
           username: ${{ env.REGISTRY_USER }}
           password: ${{ secrets.REGISTRY_TOKEN }}
 
@@ -61,7 +70,7 @@ jobs:
           context: .
           push: true
           tags: |
-            ${{ env.REGISTRY_HOST }}/${{ env.REGISTRY_OWNER }}/${{ env.REGISTRY_REPO }}:${{ steps.meta.outputs.version_tagname }}
-            ${{ env.REGISTRY_HOST }}/${{ env.REGISTRY_OWNER }}/${{ env.REGISTRY_REPO }}:latest
+            ${{ steps.meta.outputs.image }}:${{ steps.meta.outputs.version_tagname }}
+            ${{ steps.meta.outputs.image }}:latest
           cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-to: type=gha,mode=max,ignore-error=true