Make the container deployable on Dokploy

domains.json was written next to server.js, i.e. into the image filesystem,
so every redeploy discarded the domain -> glossary_id mappings. Its path is
now overridable via DOMAINS_FILE and the image defaults it to /app/data,
a subdirectory so that mounting a volume there does not shadow server.js
and index.html in /app.

Also adds a GET /health endpoint plus a Docker HEALTHCHECK that uses it. The
probe deliberately does not call DeepL, so an upstream DeepL outage does not
get the container restarted.

.gitignore is new; .env holds a real API key and was previously untracked
only by convention.

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

.dockerignore

@@ -1,6 +1,8 @@
 .env
 .env.example
+.env.testcopy
 domains.json
+data
 node_modules
 .git
 *.md