mongo-express (Docker) should also start if backend is not available

[in-fke]

mongo-express should be resilient against non-reachable mongodb and start nevertheless.
Instead

(node:7) [MONGODB DRIVER] Warning: Current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Serve
r Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
Could not connect to database using connectionString: mongodb://admin:admin@mongodb:27017/"
(node:7) UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [mongodb:27017] on first connect [Error: connect ECONNREFUSED 172.19
.0.3:27017
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1144:16) {
  name: 'MongoNetworkError'
}]
    at Pool.<anonymous> (/node_modules/mongodb/lib/core/topologies/server.js:441:11)
    at Pool.emit (events.js:314:20)
    at /node_modules/mongodb/lib/core/connection/pool.js:564:14
    at /node_modules/mongodb/lib/core/connection/pool.js:1000:11
    at /node_modules/mongodb/lib/core/connection/connect.js:32:7
    at callback (/node_modules/mongodb/lib/core/connection/connect.js:300:5)
    at Socket.<anonymous> (/node_modules/mongodb/lib/core/connection/connect.js:330:7)
    at Object.onceWrapper (events.js:421:26)
    at Socket.emit (events.js:314:20)
    at emitErrorNT (internal/streams/destroy.js:92:8)
    at emitErrorAndCloseNT (internal/streams/destroy.js:60:3)
    at processTicksAndRejections (internal/process/task_queues.js:84:21)
(node:7) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch b
lock, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandl
ed-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:7) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the
Node.js process with a non-zero exit code.