Run server as PID 1 in Docker to drop idle launcher processes

The container CMD ran `npm run start-docker`, i.e. `npm-run-all check-db
update-tracker start-server`. After the one-shot tasks exit, three Node VMs
stay resident for the container lifetime (npm -> npm-run-all -> npm ->
node server.js), each holding tens of MiB of anonymous memory for nothing.

Replace it with a small start script that runs the one-shot steps and then
`exec node server.js`, so the server becomes PID 1 and no launcher wrappers
remain. On a dev 3.2.0 build under realistic load the container RSS dropped
~28% (memory.stat anon 255 MiB -> 183 MiB); migrations still apply and
/api/heartbeat serves normally.

Dockerfile

@@ -76,4 +76,4 @@ EXPOSE 3000
 ENV HOSTNAME=0.0.0.0
 ENV PORT=3000
 
-CMD ["npm", "run", "start-docker"]
+CMD ["sh", "scripts/start-docker.sh"]

scripts/start-docker.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# Container startup: run the one-shot DB check/migration and tracker update, then
+# exec the long-running server so it replaces this shell as PID 1.
+#
+# This avoids `npm-run-all` keeping idle `npm`/`npm-run-all` parent processes
+# resident for the lifetime of the container (each a full Node VM holding tens
+# of MiB of anonymous memory). `exec` leaves a single `node server.js` process.
+#
+# PATH is extended so check-db.js's `prisma migrate deploy` resolves the prisma
+# CLI from node_modules/.bin the way `npm run` would have injected it.
+set -e
+export PATH="$PWD/node_modules/.bin:$PATH"
+
+node scripts/check-db.js
+node scripts/update-tracker.js
+exec node server.js