move cache warmup into readiness check

this will mark the pod "ready", once the cache is warm.
This will remove the penality for the very first to this service, because
it will not receive any traffic until the cache is warm.

spec/file_caching_test.js

@@ -44,6 +44,7 @@ describe('File caching service', () => {
       '/main.css': (req, res) => res.setHeader('content-type', 'text/css').status(200).send('.foo { color: #000; }')
 
     })
+    await request(app).get('/ready')
   })
 
   afterEach(() => {

src/createApp.js

@@ -35,6 +35,15 @@ export function createApp () {
   const httpLogger = pinoHttp({ logger, autoLogging: { ignorePaths } })
 
   const healthCheck = new health.HealthChecker()
+  const readinessCheck = new health.ReadinessCheck('getDependencies', async function () {
+    try {
+      await getDependencies()
+    } catch (e) {
+      logger.error(`Failed to get dependencies: ${e.message}`)
+      throw e
+    }
+  })
+  healthCheck.registerReadinessCheck(readinessCheck)
 
   // Application-level middleware
   app.use(httpLogger)
@@ -70,14 +79,12 @@ export function createApp () {
   })
 
   app.get('/', async (req, res, next) => {
-    await getDependencies()
     const { 'content-type': contentType, content } = fileCache.get('/index.html')
     if (content) return res.setHeader('content-type', contentType).status(200).send(content)
     next()
   })
 
   app.use(async (req, res, next) => {
-    await getDependencies()
     const { 'content-type': contentType, content } = fileCache.get(req.path)
     if (content) return res.setHeader('content-type', contentType).status(200).send(content)
     next()