Add Gitea Actions workflow to publish image to internal registry

Builds linux/amd64 + linux/arm64 on push to main and monthly,
pushes to gitea.technocloud.ee/technocloud-public/docker-state-exporter:latest.
Mirrors the pattern used by the other technocloud-services projects.

.gitea/workflows/build.yml

@@ -0,0 +1,52 @@
+name: Build Docker Image
+
+on:
+  push:
+    branches:
+      - main
+  schedule:
+    - cron: '@monthly'
+
+jobs:
+  build-and-deploy-image:
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    env:
+      DOCKER_ORG: technocloud-public
+      DOCKER_LATEST: latest
+      RUNNER_TOOL_CACHE: /toolcache
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          github-server-url: https://gitea.technocloud.ee
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker BuildX
+        uses: docker/setup-buildx-action@v3
+        with:
+          platforms: linux/amd64,linux/arm64
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v3
+        with:
+          registry: gitea.technocloud.ee
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Get Meta
+        id: meta
+        run: |
+          echo REPO_NAME=$(echo ${GITHUB_REPOSITORY} | awk -F"/" '{print $2}') >> $GITHUB_OUTPUT
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: gitea.technocloud.ee/${{ env.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}:${{ env.DOCKER_LATEST }}