Add Gitea Docker build workflow

.gitea/workflows/docker.yml

@@ -0,0 +1,75 @@
+name: Docker
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - "v*"
+  workflow_dispatch:
+
+env:
+  IMAGE_NAME: fsadmin/md-to-html
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Prepare tags
+        id: meta
+        shell: sh
+        run: |
+          set -eu
+
+          if [ -z "${{ vars.REGISTRY }}" ]; then
+            echo "::error::Set the REGISTRY repository variable to your Gitea registry host."
+            exit 1
+          fi
+
+          image="${{ vars.REGISTRY }}/${IMAGE_NAME}"
+          short_sha="$(printf '%s' "${GITHUB_SHA}" | cut -c1-12)"
+
+          {
+            echo "image=${image}"
+            echo "tags<<EOF"
+            echo "${image}:${short_sha}"
+            if [ "${GITHUB_REF}" = "refs/heads/main" ]; then
+              echo "${image}:latest"
+            fi
+            if [ "${GITHUB_REF_TYPE}" = "tag" ]; then
+              echo "${image}:${GITHUB_REF_NAME}"
+            fi
+            echo "EOF"
+          } >> "${GITHUB_OUTPUT}"
+
+      - name: Login to registry
+        run: |
+          printf '%s' "${{ secrets.REGISTRY_PASSWORD }}" \
+            | docker login "${{ vars.REGISTRY }}" \
+              --username "${{ secrets.REGISTRY_USERNAME }}" \
+              --password-stdin
+
+      - name: Build and push image
+        env:
+          DOCKER_BUILDKIT: "1"
+        run: |
+          set -eu
+
+          tags=""
+          while IFS= read -r tag; do
+            tags="${tags} -t ${tag}"
+          done <<'EOF'
+          ${{ steps.meta.outputs.tags }}
+          EOF
+
+          docker build ${tags} .
+
+          while IFS= read -r tag; do
+            docker push "${tag}"
+          done <<'EOF'
+          ${{ steps.meta.outputs.tags }}
+          EOF