build: add manual build script and act_runner setup for Unraid

- docker/build-and-push.sh: build and push image to forge registry from
  any machine with Docker (run directly on Unraid terminal)
- docker/act_runner.compose.yml: Gitea act_runner setup to automate
  Docker builds via Gitea Actions on Unraid

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/build-and-push.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# Build and push nabarr Docker image to the Gitea container registry.
+# Run this script on any machine that has Docker and can reach forge.dilain.com.
+#
+# Usage:
+#   ./docker/build-and-push.sh [TAG]
+#   TAG defaults to "latest"
+#
+# Example:
+#   ./docker/build-and-push.sh
+#   ./docker/build-and-push.sh master
+
+set -e
+
+REGISTRY="forge.dilain.com"
+IMAGE="${REGISTRY}/laurent/nabarr"
+TAG="${1:-latest}"
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+
+echo "==> Building ${IMAGE}:${TAG}"
+docker build \
+  --platform linux/amd64 \
+  -t "${IMAGE}:${TAG}" \
+  -f "${REPO_ROOT}/docker/Dockerfile" \
+  "${REPO_ROOT}"
+
+echo "==> Pushing ${IMAGE}:${TAG}"
+docker push "${IMAGE}:${TAG}"
+
+echo "==> Done: ${IMAGE}:${TAG}"