infra: switch to using generic tag name and manually selecting image

.gitea/workflows/gentoo-utils.yml

@@ -7,12 +7,11 @@ on:
     branches: [master]
 
 jobs:
-  build-docker:
+  build-oci-image:
     runs-on: ubuntu-latest
     continue-on-error: true
     steps:
-      -
+      - name: Checkout repo
-        name: Checkout repo
         uses: actions/checkout@v5
         with:
           fetch-depth: 0
@@ -20,13 +19,14 @@ jobs:
       - name: Check for changes before building
         id: image-changes
         run: |
+          echo "branch_name=${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}" >> $GITEA_OUTPUT
           if ! git diff ${{ gitea.event.before }} ${{ gitea.sha }} --no-patch --exit-code .docker; then
             echo changes_detected=true >> $GITEA_OUTPUT
           else
             echo changes_detected=false >> $GITEA_OUTPUT
           fi
-      -
+          cat $GITEA_OUTPUT
-        name: Set up Docker buildx
+      - name: Set up Docker buildx
         if: steps.image-changes.outputs.changes_detected == 'true'
         uses: docker/setup-buildx-action@v3
 
@@ -49,8 +49,10 @@ jobs:
           context: "{{defaultContext}}:.docker"
 
   build:
-    runs-on: gentoo-utils
+    runs-on: brutalisk
-    needs: build-docker
+    container:
+      image: git.epenguin.net/gentoo-utils/gentoo-utils-gitea:latest
+    needs: build-oci-image
     steps:
       - name: Checkout repo
         uses: actions/checkout@v5