add CI files

.gitea/workflows/docker.yml

@@ -0,0 +1,42 @@
+name: Build gentoo-utils docker image
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - '.docker/Dockerfile'
+  pull_request:
+    paths:
+      - '.docker/Dockerfile'
+  workflow_dispatch:
+
+
+jobs:
+  build-docker:
+    runs-on: ubuntu-latest
+
+    steps:
+      -
+        name: Checkout repo
+        uses: actions/checkout@v4
+
+      -
+        name: Set up Docker buildx
+        uses: docker/setup-buildx-action@v3
+
+      -
+        name: Log in to Github Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.epenguin.net
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.CI_TOKEN }}
+
+      -
+        name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          push: true
+          tags: git.epenguin.net/${{ gitea.repository }}:latest
+          context: "{{defaultContext}}:.docker"