infra: switch to using generic runner brutalisk and manually specify container image

.gitea/workflows/gentoo-utils.yml

@@ -7,7 +7,7 @@ on:
     branches: [master]
 
 jobs:
-  build-docker:
+  build-oci-image:
     runs-on: ubuntu-latest
     continue-on-error: true
     steps:
@@ -49,8 +49,10 @@ jobs:
           context: "{{defaultContext}}:.docker"
 
   build:
-    runs-on: gentoo-utils
-    needs: build-docker
+    runs-on: brutalisk
+    container:
+      image: git.epenguin.net/gentoo-utils/gentoo-utils-gitea:latest
+    needs: build-oci-image
     steps:
       - name: Checkout repo
         uses: actions/checkout@v5