feat: add docker-compose.yml + .env.example

docker-compose.yml:
- Builds the Dockerfile, exposes :8000
- Mounts ./data (persistent) and ./hadoop-conf (ro) for cluster configs
- Forwards YARN_RESOURCE_MANAGER_URL via env var (fallback for REST client)
- Healthcheck via /openapi.json (cheap liveness probe; MCP initialize
  handshake would be more accurate but requires session-id plumbing)
- restart: unless-stopped for production
- Resource limit section commented out (server itself is lightweight)

.env.example:
- Documents the env vars compose expects
- .env is gitignored

.env.example

@@ -0,0 +1,15 @@
+# Spark Executor MCP — environment template
+# Copy to .env and edit. .env is gitignored.
+
+# YARN ResourceManager URL the REST client falls back to when a Connection
+# was saved without yarn_rm_url, or a Job lacks a snapshot (e.g. if the
+# pending record was created before yarn_rm_url was a field).
+#
+# Examples:
+#   YARN_RESOURCE_MANAGER_URL=http://yarn-rm.prod.internal:8088
+#   YARN_RESOURCE_MANAGER_URL=https://yarn-rm.staging.example.com:8088
+YARN_RESOURCE_MANAGER_URL=
+
+# Optional: JVM flags forwarded to spark-submit. Useful for proxies, custom
+# truststores, or driver memory caps.
+# SPARK_SUBMIT_OPTS=-Dhttps.proxyHost=proxy.corp -Dhttps.proxyPort=3128