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.
Omitting ruby-version entirely triggers its actual auto-detection from
.ruby-version — "file" was a mistaken guess and failed with "Unknown
engine file".
Ruby syntax check, a require_relative "app" smoke load, and JS syntax
check on every push/PR. No test suite exists yet, so these are cheap
syntax/load checks rather than behavioral tests.