Add Docker deployment via Gitea Actions host-mode runner

Containerize the app (Dockerfile, docker-compose.yaml) and add a CI
deploy job that syncs the checkout to /srv/docker/maptool and runs
docker compose directly, using the runner's new "host" execution
label to reach the host's Docker daemon and filesystem.

Dockerfile

@@ -0,0 +1,17 @@
+FROM ruby:4.0.6-slim-bookworm
+
+# build-essential: puma has a native (C) extension; harmless if the
+# platform-specific gem in Gemfile.lock already has a precompiled binary
+# matching this base image, but keeps the build working if it doesn't.
+RUN apt-get update && apt-get install --no-install-recommends -y build-essential \
+  && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY Gemfile Gemfile.lock ./
+RUN bundle install
+
+COPY . .
+
+EXPOSE 4567
+CMD ["bundle", "exec", "puma", "-C", "config/puma.rb"]