Docker: Use new /health endpoint for HEALTHCHECK

src/bin/etherpad-healthcheck

@@ -0,0 +1,26 @@
+#!/usr/bin/env node
+
+// Checks the health of Etherpad by visiting http://localhost:9001/health. Returns 0 on success, 1
+// on error as required by the Dockerfile HEALTHCHECK instruction.
+
+'use strict';
+
+// As of v14, Node.js does not exit when there is an unhandled Promise rejection. Convert an
+// unhandled rejection into an uncaught exception, which does cause Node.js to exit.
+process.on('unhandledRejection', (err) => { throw err; });
+
+const assert = require('assert').strict;
+const superagent = require('superagent');
+
+(async () => {
+  const res = await superagent.get('http://localhost:9001/health')
+      .accept('application/health+json')
+      .buffer(true)
+      .parse(superagent.parse['application/json']);
+  assert(res.ok, `Unexpected HTTP status: ${res.status}`);
+  assert.equal(res.type, 'application/health+json');
+  const {body: {status} = {}} = res;
+  assert(status != null);
+  assert.equal(typeof status, 'string');
+  assert(['pass', 'ok', 'up'].includes(status.toLowerCase()), `Unexpected status: ${status}`);
+})();