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>

Dockerfile

@@ -7,10 +7,21 @@ FROM node:18-alpine
 WORKDIR /app
 COPY server.js index.html package.json ./
 
-# DEEPL_API_KEY is passed at "docker run" time (via -e or --env-file), not
-# baked into the image. domains.json should be bind-mounted so glossary
-# data survives container restarts/recreation (see README).
+# DEEPL_API_KEY is passed at runtime (docker run -e / --env-file, or the
+# Dokploy "Environment" tab), never baked into the image.
+#
+# domains.json holds which DeepL glossary_id belongs to which domain and
+# direction. It must live on a mounted volume, otherwise every redeploy
+# builds a fresh filesystem and the mappings are lost. /app/data is a
+# subdirectory so mounting it does not shadow the app files in /app.
 ENV PORT=3000
+ENV DOMAINS_FILE=/app/data/domains.json
+RUN mkdir -p /app/data
+VOLUME ["/app/data"]
+
 EXPOSE 3000
 
+HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
+  CMD wget -qO- http://127.0.0.1:3000/health || exit 1
+
 CMD ["node", "server.js"]