Backend should "wait" for the database container to be ready

The error No Living connections, Error: No Living connections, which happens when backend container is started before database is ready to receive its requests, can be (is) confusing, especially for new members in the project.

Error:

backend_1 | [ERROR] Failed getting alias map:, No Living connections, Error: No Living connections

backend_1   | Elasticsearch ERROR: 2019-02-05T13:02:19Z
backend_1   |   Error: Request error, retrying
backend_1   |   GET http://database:9200/_alias => connect ECONNREFUSED 172.19.0.2:9200
backend_1   |       at Log.error (/app/node_modules/elasticsearch/src/lib/log.js:226:56)
backend_1   |       at checkRespForFailure (/app/node_modules/elasticsearch/src/lib/transport.js:259:18)
backend_1   |       at HttpConnector.<anonymous> (/app/node_modules/elasticsearch/src/lib/connectors/http.js:163:7)
backend_1   |       at ClientRequest.wrapper (/app/node_modules/lodash/lodash.js:4935:19)
backend_1   |       at ClientRequest.emit (events.js:182:13)
backend_1   |       at Socket.socketErrorListener (_http_client.js:392:9)
backend_1   |       at Socket.emit (events.js:182:13)
backend_1   |       at emitErrorNT (internal/streams/destroy.js:82:8)
backend_1   |       at emitErrorAndCloseNT (internal/streams/destroy.js:50:3)
backend_1   |       at process._tickCallback (internal/process/next_tick.js:63:19)
backend_1   | 
backend_1   | [ERROR] Failed getting alias map:, No Living connections, Error: No Living connections
backend_1   |     at sendReqWithConnection (/app/node_modules/elasticsearch/src/lib/transport.js:226:15)
backend_1   |     at next (/app/node_modules/elasticsearch/src/lib/connection_pool.js:214:7)
backend_1   |     at process._tickCallback (internal/process/next_tick.js:61:11)

Possible solution:

healthcheck (docs)

cc: @anselmstordeur @krlwlfrt @abcdev