Intermittent slow response from Dockerized NodeJS app

I have a simple NodeJS app on top of ElasticSearch (which runs on a separate machine). When I start the app using pm2 and sequentially do some random requests, performance is fine. However, when I Dockerize the app, about one in ten responses is very slow (about 5 seconds instead of 50 ms).

This is my pm2 config:

{
    "apps": [
        {
            "name": "api",
            "script": "./app/server.js",
            "merge_logs": true,
            "max_restarts": 20,
            "instances": 1,
            "max_memory_restart": "1G",
            "env": {
                "PORT": 5000,
                "NODE_ENV": "production"
            }
        }
    ]
}

And this is the Dockerfile:

FROM alpine:3.6

ENV srcdir /src
RUN mkdir ${srcdir}
WORKDIR ${srcdir}

RUN apk add --update nodejs nodejs-npm && npm install npm@latest -g

ADD package.json ${srcdir}
RUN npm install --production
RUN npm install -g pm2

ADD . ${srcdir}

EXPOSE 5000

CMD pm2 start --no-daemon processes.json

via user3170702