ci: replace docker/login-action with direct docker login using github.token

Fixes CAR-985: docker/login-action@v3 fails with Gitea automatic
Actions token (returns 'unauthorized' on git.farh.net/v2/).

Replace the docker/login-action@v3 step with a direct docker login
using github.token piped via stdin. The github.token context variable
accesses the automatic Actions token via a different code path that
works with Gitea's built-in container registry.

Changes:
- .github/workflows/build-dev.yml: replace docker/login-action with
  direct 'echo github.token | docker login'
- .github/workflows/build-prod.yml: same replacement
- .gitea/workflows/build-dev.yml: same replacement
- .gitea/workflows/build-prod.yml: same replacement

cc @cpfarhood

build-prod.yml

@@ -0,0 +1,44 @@
+name: "Build: Production"
+
+on:
+  push:
+    branches: [local]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Gitea Registry
+        run: echo "${{ github.token }}" | docker login git.farh.net -u "${{ github.actor }}" --password-stdin
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: git.farh.net/farhoodlabs/paperclip
+          tags: |
+            type=raw,value=latest
+            type=sha,prefix=
+            type=semver,pattern={{version}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: .farhoodlabs/Dockerfile
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          no-cache: true