Default registry username to Gitea actor

.gitea/workflows/build-ci-image.yml

@@ -11,6 +11,8 @@ on:
 env:
   REGISTRY_HOST: git.neosisyphus.com
   REGISTRY_IMAGE: git.neosisyphus.com/evol3d/evol-testbed
+  # Optional. If this secret is not set, the workflow uses GITHUB_ACTOR.
+  REGISTRY_USERNAME: ${{ secrets.REGISTRY_USERNAME }}
 
 jobs:
   image:
@@ -21,8 +23,18 @@ jobs:
 
       - name: Log in to container registry
         run: |
+          set -eu
+          username="${REGISTRY_USERNAME:-${GITHUB_ACTOR:-}}"
+          if [ -z "$username" ]; then
+            echo "REGISTRY_USERNAME secret is empty and GITHUB_ACTOR is not set" >&2
+            exit 1
+          fi
+          if [ -z "${{ secrets.REGISTRY_PASSWORD }}" ]; then
+            echo "REGISTRY_PASSWORD secret is empty. Set it to a Gitea access token with package write permission." >&2
+            exit 1
+          fi
           echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login "$REGISTRY_HOST" \
-            -u "${{ secrets.REGISTRY_USERNAME }}" \
+            -u "$username" \
             --password-stdin
 
       - name: Build CI image